Check in tree-dce enh to trunk

From-SVN: r135463

8 files changed, 798 insertions, 7 deletions

diff --git a/ChangeLog b/ChangeLog
index a28e29c..0b51053 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,14 @@
+
+2008-05-17 Xinliang David Li   <davidxl@google.com>
+
+        * gcc/tree-ssa-dce.c : conditional dead call elimination
+        * gcc/opts.c         : enable the optimization at >=O2
+        * gcc/common.opt      : new flag for control the optimization
+        * gcc/doc/invoke.texi : documentation change
+        * gcc/testsuite/gcc.dg/cdce1.c : new test case
+        * gcc/testsuite/gcc.dg/cdce2.c : new test case
+        *MAINTAINERS  : Add myself (write after approval) 
+
 2008-05-15  Janus Weil  <janus@gcc.gnu.org>
 
 	* MAINTAINERS (Write After Approval): Add myself.
diff --git a/MAINTAINERS b/MAINTAINERS
index 23f98fa..b116397 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -372,6 +372,7 @@ Manuel López-Ibáñez                             manu@gcc.gnu.org
 Dave Love					d.love@dl.ac.uk
 Martin v. Löwis					loewis@informatik.hu-berlin.de
 H.J. Lu						hjl.tools@gmail.com
+Xinliang David Li                               davidxl@google.com
 William Maddox					maddox@google.com
 Ziga Mahkovec					ziga.mahkovec@klika.si
 Simon Martin					simartin@users.sourceforge.net
diff --git a/gcc/common.opt b/gcc/common.opt
index 4a44cd6..c7c6f49 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1259,6 +1259,10 @@ fweb
 Common Report Var(flag_web) Init(2) Optimization
 Construct webs and split unrelated uses of single variable
 
+ftree-builtin-dce
+Common Report Var(flag_tree_builtin_dce) Init(0) Optimization
+Enable conditional dead code elimination for builtin calls.
+
 fwhole-program
 Common Report Var(flag_whole_program) Init(0) Optimization
 Perform whole program optimizations
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index e515a0f..0fdadd2 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -353,7 +353,7 @@ Objective-C and Objective-C++ Dialects}.
 -fsignaling-nans -fsingle-precision-constant -fsplit-ivs-in-unroller @gol
 -fsplit-wide-types -fstack-protector -fstack-protector-all @gol
 -fstrict-aliasing -fstrict-overflow -fthread-jumps -ftracer -ftree-ccp @gol
--ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-dce @gol
+-ftree-ch -ftree-copy-prop -ftree-copyrename -ftree-dce -ftree-builtin-dce@gol
 -ftree-dominator-opts -ftree-dse -ftree-fre -ftree-loop-im @gol
 -ftree-loop-distribution @gol
 -ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize @gol
@@ -5157,6 +5157,7 @@ compilation time.
 -ftree-ch @gol
 -ftree-copyrename @gol
 -ftree-dce @gol
+-ftree-builtin-dce @gol
 -ftree-dominator-opts @gol
 -ftree-dse @gol
 -ftree-fre @gol
@@ -5873,6 +5874,12 @@ enabled by default at @option{-O2} and higher.
 Perform dead code elimination (DCE) on trees.  This flag is enabled by
 default at @option{-O} and higher.
 
+@item -ftree-builtin-dce
+@opindex ftree-builtin-dce
+Perform conditional dead code elimination (DCE) on builtin calls that
+may set errno but are otherwise side-effect free.  This flag is enabled by
+default at @option{-O} and higher.
+
 @item -ftree-dominator-opts
 @opindex ftree-dominator-opts
 Perform a variety of simple scalar cleanups (constant/copy
diff --git a/gcc/opts.c b/gcc/opts.c
index 7add8d3..b03c5a7 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -886,6 +886,7 @@ decode_options (unsigned int argc, const char **argv)
       flag_reorder_functions = 1;
       flag_tree_store_ccp = 1;
       flag_tree_vrp = 1;
+      flag_tree_builtin_dce = 1;
 
       if (!optimize_size)
 	{
diff --git a/gcc/testsuite/gcc.dg/cdce1.c b/gcc/testsuite/gcc.dg/cdce1.c
new file mode 100644
index 0000000..8e29fc4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cdce1.c
@@ -0,0 +1,81 @@
+/* { dg-do  run } */
+/* { dg-options "-O2 -fdump-tree-dce1-details  -lm" } */
+/* { dg-message  "note: function call is shrink-wrapped into error conditions\." "Missing conditional dce" {target "*-*-*"} 15 } */
+ 
+#include <stdlib.h>
+#include <math.h>
+#include <errno.h>
+#include <stdio.h>
+int total_err_count = 0;
+double foo_opt(int x, double y) __attribute__((noinline));
+double foo_opt(int x, double y)
+{
+    double yy = 0;
+    errno = 0;
+    yy = pow(x,y);
+    return 0;
+}
+
+double foo(int x, double y) __attribute__((noinline));
+double foo(int x, double y)
+{
+    double yy = 0;
+    errno = 0;
+    yy = pow(x,y);
+    return yy;
+}
+
+int test(double (*fp)(int x, double y))
+{
+   int i,x;
+
+   x = 127; 
+   for (i = 30; i < 300; i++)
+   {  
+      fp(x,i);
+      if (errno)
+         total_err_count ++;
+   }
+
+   x = -300; 
+   for (i = 100; i < 300; i++)
+   {  
+      fp(x,i);
+      if (errno)
+         total_err_count ++;
+   }
+
+   x = 65577;
+   for (i = 60; i < 200; i++)
+   {  
+      fp(x,i);
+      if (errno)
+         total_err_count ++;
+   }
+
+   x = 65577*127;
+   for (i = 1; i < 100; i++)
+   {  
+      fp(x,i);
+      if (errno)
+         total_err_count ++;
+   }
+
+   return total_err_count;
+}
+
+int main()
+{
+  int en1, en2;
+  total_err_count = 0;
+  en1 = test(foo_opt);
+  total_err_count = 0;
+  en2 = test(foo);
+  
+  printf("total number of errors = %d, %d\n", en1, en2);
+  if (en1 != en2)
+     abort();
+
+  return 0;
+
+}
diff --git a/gcc/testsuite/gcc.dg/cdce2.c b/gcc/testsuite/gcc.dg/cdce2.c
new file mode 100644
index 0000000..8761872
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/cdce2.c
@@ -0,0 +1,56 @@
+/* { dg-do  run } */
+/* { dg-options "-O2 -fdump-tree-dce1-details  -lm" } */
+/* { dg-message  "note: function call is shrink-wrapped into error conditions\." "Missing conditional dce" {target "*-*-*"} 15 } */
+ 
+#include <stdlib.h>
+#include <math.h>
+#include <errno.h>
+#include <stdio.h>
+int total_err_count = 0;
+double foo_opt(double y) __attribute__((noinline));
+double foo_opt(double y)
+{
+    double yy = 0;
+    errno = 0;
+    yy = log(y);
+    return 0;
+}
+
+double foo(double y) __attribute__((noinline));
+double foo(double y)
+{
+    double yy = 0;
+    errno = 0;
+    yy = log(y);
+    return yy;
+}
+
+int test(double (*fp)(double y) )
+{
+   int i,x;
+
+   for (i = -100; i < 100; i++)
+   {  
+      fp(i);
+      if (errno)
+         total_err_count ++;
+   }
+
+  return total_err_count;
+}
+
+int main()
+{
+  int en1, en2;
+  double yy;
+  total_err_count = 0;
+  en1 = test(foo_opt);
+  total_err_count = 0;
+  en2 = test(foo);
+
+  if (en1 != en2)
+     abort();
+
+  return 0;
+
+}
diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index 95457eb..d1f1c06 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -48,7 +48,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "coretypes.h"
 #include "tm.h"
 #include "ggc.h"
-
+#include "diagnostic.h"
+#include "toplev.h"
 /* These RTL headers are needed for basic-block.h.  */
 #include "rtl.h"
 #include "tm_p.h"
@@ -75,7 +76,9 @@ static struct stmt_stats
   int removed_phis;
 } stats;
 
-static VEC(tree,heap) *worklist;
+
+static VEC (tree, heap) *worklist;
+static VEC (tree, heap) *cond_dead_built_in_calls;
 
 /* Vector indicating an SSA name has already been processed and marked
    as necessary.  */
@@ -199,6 +202,189 @@ find_all_control_dependences (struct edge_list *el)
 
 #define NECESSARY(stmt)		stmt->base.asm_written_flag
 
+/*  Conditional dead call elimination 
+
+   Some builtin functions can set errno on error conditions, but they
+   are otherwise pure. If the result of a call to such a function is 
+   not used, the compiler can still not eliminate the call without 
+   powerful interprocedural analysis to prove that the errno is not
+   checked. However, if the conditions under which the error occurs
+   are known, compiler can conditionally dead code eliminate the calls
+   by shrink-wrapping the semi-dead calls into the error condition:
+      
+        built_in_call(args) 
+          ==>
+        if (error_cond(args))
+             built_in_call(args)
+
+   ( An actual simple exampl is :
+         log (x);   // Mostly dead call
+     ==>
+         if (x < 0)
+             log(x);
+     With this change, call to log(x) is effectively eliminated, as
+     in majority of the cases, log won't be called with x out of 
+     range. The branch is totally predicatible, so the branch cost
+     is low.  Such optimization improves the performance of  
+     an important application in a big search company by 20% )
+   
+   Note that library functions are not supposed to clear errno to zero without
+   error.
+   
+   In this implementation, only 'pow' and 'log' are handled. ('sin' and 'cos' 
+   seem to be wrongly handled by gcc -- they are treated as not touching errno
+   which is not correct.)
+   
+   The condition wrapping the builtin call is conservatively set to avoid too
+   aggressive (wrong) shrink wrapping. The optimization is called conditional
+   dead call elimination because the call is eliminated under the condition 
+   that the input arguments would not lead to domain or range error (for 
+   instance when x <= 0 for a log(x) call), however the chances that the error
+   condition is hit is very low (those builtin calls which are conditionally 
+   dead are usually part of the C++ abstraction penalty exposed after 
+   inlining).  */
+
+
+static bool
+check_pow (tree pow_call)
+{
+  tree base, expn;
+  enum tree_code bc, ec;
+
+  gcc_assert (TREE_CODE (pow_call) == CALL_EXPR);
+  if (call_expr_nargs (pow_call) != 2)
+    return false;
+
+  base = CALL_EXPR_ARG (pow_call, 0);
+  expn = CALL_EXPR_ARG (pow_call, 1);
+
+  bc = TREE_CODE (base);
+  ec = TREE_CODE (expn);
+ 
+  gcc_assert (TREE_CODE_CLASS (bc) != tcc_constant 
+             || bc == REAL_CST);
+  gcc_assert (TREE_CODE_CLASS (ec) != tcc_constant 
+             || ec == REAL_CST);
+
+  /* Folding candidates are not interesting.  */
+  if (ec == REAL_CST && bc == REAL_CST)
+    return false;
+
+  if (bc == REAL_CST)
+   {
+     /* Only handle a fixed range of constant.  */
+     REAL_VALUE_TYPE mv;
+     REAL_VALUE_TYPE bcv = TREE_REAL_CST (base);
+     if (REAL_VALUES_EQUAL (bcv, dconst1))
+       return false;
+     if (REAL_VALUES_LESS (bcv, dconst1))
+       return false;
+     real_from_integer (&mv, VOIDmode,256,0,1);
+     if (REAL_VALUES_LESS (mv, bcv))
+       return false;
+     return true;
+   }
+  else if (bc == SSA_NAME)
+   {
+     tree def, nm, rhs, rhs0, var, typ;
+     int sz;
+
+     def = SSA_NAME_DEF_STMT (base);
+     if (TREE_CODE (def) != GIMPLE_MODIFY_STMT)
+       return false;
+
+     nm = GIMPLE_STMT_OPERAND (def,0);
+     gcc_assert (TREE_CODE (nm) == SSA_NAME);
+     if (nm != base) 
+       return false;
+
+     rhs = GIMPLE_STMT_OPERAND (def,1);
+     
+     if (TREE_CODE (rhs) != FLOAT_EXPR)
+       return false;
+     rhs0 = TREE_OPERAND (rhs,0);
+
+     if (TREE_CODE (rhs0) != SSA_NAME)
+       return false;
+
+     var = SSA_NAME_VAR (rhs0);
+     if (TREE_CODE (var) != VAR_DECL &&
+         TREE_CODE (var) != PARM_DECL)
+       return false;
+
+     typ = TREE_TYPE (var);
+     if (TREE_CODE (typ) != INTEGER_TYPE)
+       return false;
+     sz = int_size_in_bytes (typ);
+     if (sz == -1 || sz > INT_TYPE_SIZE) 
+       return false;
+
+     return true;
+   }
+  else
+    return false;
+}
+
+static bool
+check_log (tree log_call)
+{
+  tree arg_typ;
+  gcc_assert (TREE_CODE (log_call) == CALL_EXPR);
+  if (call_expr_nargs (log_call) != 1)
+    return false;
+
+  arg_typ = TREE_TYPE (CALL_EXPR_ARG (log_call, 0));
+  if (!is_gimple_reg_type (arg_typ))
+    return false;
+  return true;
+}
+
+
+/*  A helper function to determine if a builtin function
+    call is a candidate for conditional DCE.  */
+
+static bool
+is_unnecessary_except_errno_call (tree call)
+{
+  tree fn;
+  bool is_unnecessary_except_errno = false;
+  enum built_in_function fnc;
+
+  if (!flag_tree_builtin_dce) 
+    return false;
+
+  gcc_assert (call);
+  gcc_assert (!DECL_P (call));
+  gcc_assert (TREE_CODE (call) == CALL_EXPR);
+
+  fn = get_callee_fndecl (call);
+  if (!fn || !DECL_BUILT_IN (fn)) 
+    return false;
+
+  fnc = DECL_FUNCTION_CODE (fn);
+  switch (fnc)
+   {
+     CASE_FLT_FN (BUILT_IN_POW): 
+         if (check_pow (call))
+           is_unnecessary_except_errno = true;
+     break;
+
+     CASE_FLT_FN (BUILT_IN_LOG):
+         if (check_log (call))
+           is_unnecessary_except_errno = true;
+     break;
+     default : 
+       is_unnecessary_except_errno = false;
+     break;
+   }
+
+  if (!is_unnecessary_except_errno) 
+    return false;
+
+  return is_unnecessary_except_errno;
+}
+
+
 /* If STMT is not already marked necessary, mark it, and add it to the
    worklist if ADD_TO_WORKLIST is true.  */
 static inline void
@@ -244,6 +430,13 @@ mark_operand_necessary (tree op)
   if (NECESSARY (stmt) || IS_EMPTY_STMT (stmt))
     return;
 
+  if (dump_file && (dump_flags & TDF_DETAILS))
+   {
+     fprintf (dump_file, " Marked as necessary: ");
+     print_generic_stmt (dump_file, stmt, TDF_SLIM);
+     fprintf (dump_file, "\n");
+   }
+
   NECESSARY (stmt) = 1;
   VEC_safe_push (tree, heap, worklist, stmt);
 }
@@ -294,15 +487,16 @@ mark_stmt_if_obviously_necessary (tree stmt, bool aggressive)
       /* Most, but not all function calls are required.  Function calls that
 	 produce no result and have no side effects (i.e. const pure
 	 functions) are unnecessary.  */
-      if (TREE_SIDE_EFFECTS (stmt))
-	mark_stmt_necessary (stmt, true);
+      if (TREE_SIDE_EFFECTS (stmt)) 
+        mark_stmt_necessary (stmt, true);
+      
       return;
 
     case GIMPLE_MODIFY_STMT:
       op = get_call_expr_in (stmt);
       if (op && TREE_SIDE_EFFECTS (op))
 	{
-	  mark_stmt_necessary (stmt, true);
+          mark_stmt_necessary (stmt, true);
 	  return;
 	}
 
@@ -523,6 +717,421 @@ propagate_necessity (struct edge_list *el)
     }
 }
 
+/*  Method to generate conditional statements for guarding condionally
+    dead calls to pow. One or more statements can be generated for 
+    each logical condition. Statement groups of different conditions
+    are separated by a NULL tree.  */
+static void
+gen_conditions_for_pow (tree pow_call, enum built_in_function fnc, 
+                        VEC (tree, heap)* conds, unsigned * nconds)
+{
+  tree base, expn;
+  enum tree_code bc, ec;
+  gcc_assert (TREE_CODE (pow_call) == CALL_EXPR);
+  gcc_assert (call_expr_nargs (pow_call) == 2);
+  gcc_assert (fnc == BUILT_IN_POW);
+
+  *nconds = 0;
+
+  base = CALL_EXPR_ARG (pow_call, 0);
+  expn = CALL_EXPR_ARG (pow_call, 1);
+
+  bc = TREE_CODE (base);
+  ec = TREE_CODE (expn);
+  
+  gcc_assert (TREE_CODE_CLASS (bc) != tcc_constant ||
+             bc == REAL_CST);
+  gcc_assert (TREE_CODE_CLASS (ec) != tcc_constant ||
+             ec == REAL_CST);
+
+  if (bc == REAL_CST)
+   {
+     tree float_typ, max_exp_real_cst;
+     tree temp, tempn, tempc, tempcn, stmt1, stmt2, stmt3;
+     REAL_VALUE_TYPE mv;
+
+     /* See candidate selection in check_pow. 
+        Since the candidates have a given range
+        of base values, the guard code generated
+        for such calls: pow(const,y) are simple:
+           if ( y > max_y )
+               pow(const, y);
+        max_y can be computed separately for each
+        const base, but in this implemetation, we
+        choose to compute it using the max base
+        in the allowed range.  */
+
+     REAL_VALUE_TYPE bcv = TREE_REAL_CST (base);
+     gcc_assert (!REAL_VALUES_EQUAL (bcv, dconst1));
+     gcc_assert (!REAL_VALUES_LESS (bcv, dconst1));
+     real_from_integer (&mv, VOIDmode,256,0,1),
+     gcc_assert (!REAL_VALUES_LESS (mv, bcv));
+     float_typ = TREE_TYPE (expn); 
+
+     max_exp_real_cst = build_real (float_typ, mv);
+     temp = create_tmp_var (float_typ, "DCE_COND");
+     stmt1 = build_gimple_modify_stmt (temp, expn);
+     tempn = make_ssa_name (temp, stmt1);
+     GIMPLE_STMT_OPERAND (stmt1, 0) = tempn;
+
+     tempc = create_tmp_var (boolean_type_node, "DCE_COND_TEST");
+     stmt2 = build_gimple_modify_stmt (tempc, 
+                                       build2 (GT_EXPR, 
+                                               boolean_type_node, 
+                                               tempn, max_exp_real_cst));
+     tempcn = make_ssa_name (tempc, stmt2);
+     GIMPLE_STMT_OPERAND (stmt2, 0) = tempcn;
+
+     stmt3 = build3 (COND_EXPR, void_type_node,
+                     tempcn, NULL_TREE, NULL_TREE);
+     VEC_safe_push (tree, heap, conds, stmt1);
+     VEC_safe_push (tree, heap, conds, stmt2);
+     VEC_safe_push (tree, heap, conds, stmt3);
+     (*nconds)++;
+
+   }
+  else if (bc == SSA_NAME)
+   {
+     tree def, nm, rhs, rhs0, var, int_typ, float_typ;
+     tree max_exp_cst, max_exp_real_cst;
+     tree temp1, temp1n, temp2, temp2n, temp2c, temp2cn; 
+     tree cst0, stmt1, stmt2, stmt3;
+     int sz, max_exp;
+
+     /* Generate error condition code for pow calls with
+        non constant base value. The candidates selected
+        have their base argument value converted from
+        integer (see check_pow) value (1,2,4 bytes), and
+        the max exp value is computed based on the size
+        of the integer type.  The code below first does
+        sanity check and then does code generation.  */
+
+     def = SSA_NAME_DEF_STMT (base);
+     gcc_assert (TREE_CODE (def) == GIMPLE_MODIFY_STMT);
+
+     nm = GIMPLE_STMT_OPERAND (def,0);
+     gcc_assert (TREE_CODE (nm) == SSA_NAME);
+     gcc_assert (nm == base); 
+
+     rhs = GIMPLE_STMT_OPERAND (def,1);
+     
+     gcc_assert (TREE_CODE (rhs) == FLOAT_EXPR);
+     rhs0 = TREE_OPERAND (rhs,0);
+     gcc_assert (TREE_CODE (rhs0) == SSA_NAME);
+
+     var = SSA_NAME_VAR (rhs0);
+     gcc_assert (TREE_CODE (var) == VAR_DECL 
+                 || TREE_CODE (var) == PARM_DECL);
+     
+     int_typ = TREE_TYPE (var);
+     gcc_assert (TREE_CODE (int_typ) == INTEGER_TYPE);
+
+     sz = int_size_in_bytes (int_typ);
+     gcc_assert (sz > 0 && sz <= INT_TYPE_SIZE) ;
+
+
+     float_typ = TREE_TYPE (SSA_NAME_VAR (expn)); 
+     if (sz == 1)
+       max_exp = 128;
+     else if (sz == 2)
+       max_exp = 64;
+     else 
+      {
+        gcc_assert (sz == 4);
+        max_exp = 32;
+      }
+     max_exp_cst = build_int_cst (integer_type_node, max_exp);
+     max_exp_real_cst = build_real_from_int_cst (float_typ, max_exp_cst);
+     
+     /* For pow ((dobule)x,y), generate the following conditions:
+      cond 1:
+        temp1 = x;
+        if (temp1 <= 0)
+
+      cond 2:
+        temp2 = y;
+        if (temp2 > max_exp_real_cst)  */
+
+     temp2 = create_tmp_var (float_typ, "DCE_COND2");
+     stmt1 = build_gimple_modify_stmt (temp2, expn);
+     temp2n = make_ssa_name (temp2, stmt1);
+     GIMPLE_STMT_OPERAND (stmt1,0) = temp2n;
+
+     temp2c = create_tmp_var (boolean_type_node, "DCE_COND2_TEST");
+     stmt2 = build_gimple_modify_stmt (temp2c, 
+                                       build2 (GT_EXPR, 
+                                               boolean_type_node, 
+                                               temp2n, max_exp_real_cst));
+     temp2cn = make_ssa_name (temp2c, stmt2);
+     GIMPLE_STMT_OPERAND (stmt2, 0) = temp2cn;
+
+     stmt3 = build3 (COND_EXPR, void_type_node,
+                     temp2cn, NULL_TREE, NULL_TREE);
+     VEC_safe_push (tree, heap, conds, stmt1);
+     VEC_safe_push (tree, heap, conds, stmt2);
+     VEC_safe_push (tree, heap, conds, stmt3);
+     (*nconds)++;
+
+     /* now a seperator*/
+     VEC_safe_push (tree, heap, conds, NULL);
+
+     temp1 = create_tmp_var (int_typ, "DCE_COND1");
+     cst0 = build_int_cst (int_typ, 0);
+     stmt1 = build_gimple_modify_stmt (temp1, rhs0);
+     temp1n = make_ssa_name (temp1, stmt1);
+     GIMPLE_STMT_OPERAND (stmt1,0) = temp1n;
+     stmt2 = build3 (COND_EXPR, void_type_node,
+                     build2 (LE_EXPR, boolean_type_node, temp1n, cst0),
+                     NULL_TREE, NULL_TREE);
+
+     VEC_safe_push (tree, heap, conds, stmt1);
+     VEC_safe_push (tree, heap, conds, stmt2);
+     (*nconds)++;
+
+   }
+  else
+    gcc_unreachable ();
+}
+
+/*  The method to generate error condition guard code for log(x)
+    calls.  */
+static void
+gen_conditions_for_log (tree call, enum built_in_function fnc, 
+                        VEC (tree, heap)* conds, unsigned * nconds)
+{
+  tree arg, cst0, temp, tempn, tempc, tempcn, stmt1, stmt2, stmt3;
+  gcc_assert (TREE_CODE (call) == CALL_EXPR);
+  gcc_assert (fnc == BUILT_IN_LOG || fnc == BUILT_IN_LOGF || fnc == BUILT_IN_LOGL);
+
+  *nconds = 0;
+
+  /* for log(x), 
+   Generate condition
+
+   temp = x
+   if (x <= 0)
+  */
+  arg = CALL_EXPR_ARG (call, 0);
+  cst0 = build_real (TREE_TYPE (arg), dconst0);
+  temp = create_tmp_var (TREE_TYPE (arg), "DCE_COND");
+  stmt1 = build_gimple_modify_stmt (temp, arg);
+  tempn = make_ssa_name (temp, stmt1);
+  GIMPLE_STMT_OPERAND (stmt1,0) = tempn;
+
+  tempc = create_tmp_var (boolean_type_node, "DCE_COND_TEST");
+  stmt2 = build_gimple_modify_stmt (tempc, 
+                                    build2 (LE_EXPR, 
+                                            boolean_type_node, 
+                                            tempn, cst0));
+  tempcn = make_ssa_name (tempc, stmt2);
+  GIMPLE_STMT_OPERAND (stmt2, 0) = tempcn;
+
+  stmt3 = build3 (COND_EXPR, void_type_node, tempcn,
+                       NULL_TREE, NULL_TREE);
+
+  VEC_safe_push (tree, heap, conds, stmt1);
+  VEC_safe_push (tree, heap, conds, stmt2);
+  VEC_safe_push (tree, heap, conds, stmt3);
+  (*nconds)++;
+
+}
+
+
+/*  The method to generate shrink wrap conditions for partially 
+    a dead builtin call whose return value is not used anywhere,
+    but has to be kept live due to potential error condition.  
+
+    BI_CALL:  the builtin call
+    CONDS  :  the vector of statements for condition code
+    NCODES :  the pointer to the number of logical conditions, 
+              which may be different from the length of CONDS
+              vector. Statements belonging to different logical
+              condition are separated by NULL tree in the vector
+*/
+
+static void
+gen_shrink_wrap_conditions (tree bi_call, VEC (tree, heap)* conds, unsigned int * nconds)
+{
+  tree call, fn;
+  enum built_in_function fnc; 
+  gcc_assert (nconds && conds);
+  gcc_assert (VEC_length(tree, conds) == 0);
+  gcc_assert (TREE_CODE (bi_call) == GIMPLE_MODIFY_STMT ||
+             TREE_CODE (bi_call) == CALL_EXPR);
+
+  call = bi_call;
+  if (TREE_CODE (call) == GIMPLE_MODIFY_STMT)
+    call = get_call_expr_in (bi_call);
+
+  fn = get_callee_fndecl (call);
+  gcc_assert (fn && DECL_BUILT_IN (fn)); 
+  fnc = DECL_FUNCTION_CODE (fn);
+  *nconds = 0;
+
+  switch (fnc)
+   {
+     /*CASE_FLT_FN(BUILT_IN_POW)*/
+     case BUILT_IN_POW:
+       gen_conditions_for_pow (call, fnc, conds, nconds);
+     break;
+     /*CASE_FLT_FN(BUILT_IN_LOG):*/
+     case BUILT_IN_LOG:
+     case BUILT_IN_LOGF:
+     case BUILT_IN_LOGL:
+       gen_conditions_for_log (call, fnc, conds, nconds);
+     break;
+     default : 
+       gcc_assert (0);
+     break;
+   }
+
+  gcc_assert (*nconds);
+  return;
+}
+
+
+#define ERR_PROB 0.01
+
+/*  The method to shrink wrap a partially  dead builtin call 
+    whose return value is not used anywhere, but has to be kept 
+    live due to potential error condition.  */
+static void
+shrink_wrap_one_built_in_call (tree bi_call)
+{
+  block_stmt_iterator bi_call_bsi, join_tgt_bsi;
+  basic_block bi_call_bb, join_tgt_bb, guard_bb, guard_bb0;
+  edge join_tgt_in_edge_from_call, join_tgt_in_edge_fall_thru;
+  tree join_tgt_label_decl, join_tgt_label;
+  edge bi_call_in_edge0, guard_bb_in_edge;
+  VEC (tree, heap) *conds;
+  unsigned tn_cond_stmts, nconds;
+  unsigned ci;
+  tree cond_expr = NULL;
+  tree cond_expr_start;
+  tree bi_call_label_decl;
+  tree bi_call_label;
+
+  conds = VEC_alloc (tree, heap, 10);
+  gen_shrink_wrap_conditions (bi_call, conds, &nconds);
+
+  gcc_assert (nconds > 0);
+  /* Make sure no reallocation happens. */
+  gcc_assert (VEC_length (tree, conds) <= 10);
+  gcc_assert (VEC_length (tree, conds) >= nconds);
+  bi_call_bb = bb_for_stmt (bi_call);
+
+  /* Now find the join target bb -- split 
+     bi_call_bb if needed */
+  bi_call_bsi = bsi_for_stmt (bi_call);
+
+  gcc_assert (!bsi_end_p (bi_call_bsi));
+  join_tgt_in_edge_from_call = split_block (bi_call_bb, bi_call);
+  bi_call_bsi = bsi_for_stmt (bi_call);
+
+  join_tgt_bb = join_tgt_in_edge_from_call->dest;
+  join_tgt_label_decl = create_artificial_label ();
+  join_tgt_label = build1 (LABEL_EXPR, void_type_node, join_tgt_label_decl);
+  join_tgt_bsi = bsi_start (join_tgt_bb);
+  bsi_insert_before (&join_tgt_bsi, join_tgt_label, BSI_SAME_STMT);
+
+  /* Now it is time to insert the first condtional expression 
+     into bi_call_bb and split this bb so that bi_call is 
+     shrink-wrapped*/
+  tn_cond_stmts = VEC_length (tree, conds);
+  cond_expr = NULL;
+  cond_expr_start = VEC_index (tree, conds,0);
+  for (ci = 0; ci < tn_cond_stmts; ci++)
+   {
+     tree c = VEC_index (tree, conds, ci);
+     gcc_assert ( c || ci != 0 );
+     if (!c) break;
+     bsi_insert_before (&bi_call_bsi, c, BSI_SAME_STMT);
+     cond_expr = c;
+   }
+  nconds --;
+  ci ++;
+  gcc_assert (cond_expr && TREE_CODE (cond_expr) == COND_EXPR);
+
+  /* now the label*/
+  bi_call_label_decl = create_artificial_label ();
+  bi_call_label = build1 (LABEL_EXPR, void_type_node, bi_call_label_decl);
+  bsi_insert_before (&bi_call_bsi, bi_call_label, BSI_SAME_STMT);
+
+  bi_call_in_edge0 = split_block (bi_call_bb, cond_expr);
+  bi_call_in_edge0->flags &= ~EDGE_FALLTHRU;
+  bi_call_in_edge0->flags |= EDGE_TRUE_VALUE;
+  guard_bb0 = bi_call_bb;
+  bi_call_bb = bi_call_in_edge0->dest;
+  join_tgt_in_edge_fall_thru = make_edge (guard_bb0, join_tgt_bb, EDGE_FALSE_VALUE);
+
+  bi_call_in_edge0->probability = REG_BR_PROB_BASE*ERR_PROB;
+  join_tgt_in_edge_fall_thru->probability = 
+      REG_BR_PROB_BASE - bi_call_in_edge0->probability;
+
+  /* code generation for the rest of the conditions */
+  guard_bb = guard_bb0;
+  for (; nconds > 0; )
+   {
+     unsigned ci0;
+     edge bi_call_in_edge; 
+     block_stmt_iterator guard_bsi = bsi_for_stmt (cond_expr_start);
+     ci0 = ci;
+     cond_expr_start = VEC_index (tree, conds, ci0);
+     for (; ci < tn_cond_stmts; ci++)
+      {
+        tree c = VEC_index (tree, conds, ci);
+        gcc_assert ( c || ci != ci0 );
+        if (!c) 
+          break;
+        bsi_insert_before (&guard_bsi, c, BSI_SAME_STMT);
+        cond_expr = c;
+      }
+     nconds --;
+     ci ++;
+     gcc_assert (cond_expr && TREE_CODE (cond_expr) == COND_EXPR);
+     guard_bb_in_edge = split_block (guard_bb, cond_expr);
+     guard_bb_in_edge->flags &= ~EDGE_FALLTHRU;
+     guard_bb_in_edge->flags |= EDGE_FALSE_VALUE;
+
+     bi_call_in_edge = make_edge (guard_bb, bi_call_bb, EDGE_TRUE_VALUE);
+
+     bi_call_in_edge->probability = REG_BR_PROB_BASE*ERR_PROB;
+     guard_bb_in_edge->probability = 
+         REG_BR_PROB_BASE - bi_call_in_edge->probability;
+
+   }
+
+  VEC_free (tree, heap, conds);
+  if (dump_file && (dump_flags & TDF_DETAILS))
+   {
+     location_t loc;
+     loc = EXPR_LOCATION (bi_call);
+     inform (
+         "%Hfunction call is shrink-wrapped into error conditions.",
+         &loc);
+   }
+}
+
+/*  The top level method for conditional dead code shrink 
+    wrapping transformation.  */
+
+static bool
+shrink_wrap_conditional_dead_built_in_calls (void)
+{
+  unsigned i = 0;
+  unsigned n = VEC_length (tree, cond_dead_built_in_calls);
+  if (n == 0) return false;
+
+  for (; i < n ; i++)
+   {
+     tree bi_call = VEC_index (tree,  cond_dead_built_in_calls, i);
+     shrink_wrap_one_built_in_call (bi_call);
+   }
+
+  cfg_altered = true;
+
+  return true;
+}
 
 /* Remove dead PHI nodes from block BB.  */
 
@@ -711,6 +1320,18 @@ eliminate_unnecessary_stmts (void)
 			  print_generic_stmt (dump_file, t, TDF_SLIM);
 			  fprintf (dump_file, "\n");
 			}
+
+                      if (is_unnecessary_except_errno_call (call))
+                       {
+                         if (dump_file && (dump_flags & TDF_DETAILS))
+			  {
+                            fprintf (dump_file, "Found conditional dead call: ");
+                            print_generic_stmt (dump_file, t, TDF_SLIM);
+                            fprintf (dump_file, "\n");
+                          }
+                         VEC_safe_push (tree, heap, cond_dead_built_in_calls, call);
+                       }
+
 		      push_stmt_changes (bsi_stmt_ptr (i));
 		      TREE_BLOCK (call) = TREE_BLOCK (t);
 		      bsi_replace (&i, call, false);
@@ -718,6 +1339,8 @@ eliminate_unnecessary_stmts (void)
 		      mark_symbols_for_renaming (call);
 		      pop_stmt_changes (bsi_stmt_ptr (i));
 		      release_ssa_name (oldlhs);
+
+
 		    }
 		  notice_special_calls (call);
 		}
@@ -726,6 +1349,9 @@ eliminate_unnecessary_stmts (void)
 	}
     }
 
+  something_changed |= 
+      shrink_wrap_conditional_dead_built_in_calls ();
+
   return something_changed;
 }
 
@@ -776,7 +1402,9 @@ tree_dce_init (bool aggressive)
   sbitmap_zero (processed);
 
   worklist = VEC_alloc (tree, heap, 64);
+  cond_dead_built_in_calls = VEC_alloc (tree, heap,64);
   cfg_altered = false;
+  
 }
 
 /* Cleanup after this pass.  */
@@ -799,7 +1427,9 @@ tree_dce_done (bool aggressive)
   sbitmap_free (processed);
 
   VEC_free (tree, heap, worklist);
+  VEC_free (tree, heap, cond_dead_built_in_calls);
 }
+
 
 /* Main routine to eliminate dead code.
 
@@ -841,7 +1471,7 @@ perform_tree_ssa_dce (bool aggressive)
   find_obviously_necessary_stmts (el);
 
   propagate_necessity (el);
-
+  
   something_changed |= eliminate_unnecessary_stmts ();
   something_changed |= cfg_altered;