cse.c (find_comparison_args): Update documentation.

	* cse.c (find_comparison_args): Update documentation.  Fix
	mishandling of COMPARE operations.

	* tree.def (ABS_EXPR): Add documentation.
	* fold-const.c (fold): Improve folding of ABS_EXPRs.

From-SVN: r40630

1 files changed, 47 insertions, 31 deletions

diff --git a/gcc/cse.c b/gcc/cse.c
index 38ba919..9ba7f72 100644
--- a/gcc/cse.c
+++ b/gcc/cse.c
@@ -3086,17 +3086,25 @@ find_best_addr (insn, loc, mode)
 #endif
 }
 
-/* Given an operation (CODE, *PARG1, *PARG2), where code is a comparison
-   operation (EQ, NE, GT, etc.), follow it back through the hash table and
-   what values are being compared.
+/* This routine accepts a comparison as input and attempts to return a
+   comparision that is cheaper to compute.
 
-   *PARG1 and *PARG2 are updated to contain the rtx representing the values
-   actually being compared.  For example, if *PARG1 was (cc0) and *PARG2
-   was (const_int 0), *PARG1 and *PARG2 will be set to the objects that were
-   compared to produce cc0.
+   On input, *PARG1 and *PARG2 should be set to the first and second
+   arguments to the comparison, respectively.  CODE is the comparision
+   code.  For example, if the comparison is:
 
-   The return value is the comparison operator and is either the code of
-   A or the code corresponding to the inverse of the comparison.  */
+     (ne:SI (reg:CC 24 cc)
+            (const_int 0 [0x0])))
+
+   The CODE should be NE, *PARG1 should be `(reg:CC 24 cc)' and 
+   *PARG2 should be `(const_int 0)'.
+
+   Upon return, *PARG1 and and *PARG2 may have new values, indicating
+   arguments to a cheaper comparison.  *PMODE1 and *PMODE2 will be the
+   modes that should be used for those arguments.  The return value
+   itself will be the comparison code that should be used to compare
+   *PARG1 and *PARG2 in order to obtain a value equivalent to that
+   given by the original comparison.  */
 
 static enum rtx_code
 find_comparison_args (code, parg1, parg2, pmode1, pmode2)
@@ -3187,30 +3195,38 @@ find_comparison_args (code, parg1, parg2, pmode1, pmode2)
 	  if (! exp_equiv_p (p->exp, p->exp, 1, 0))
 	    continue;
 
-	  if (GET_CODE (p->exp) == COMPARE
-	      /* Another possibility is that this machine has a compare insn
-		 that includes the comparison code.  In that case, ARG1 would
-		 be equivalent to a comparison operation that would set ARG1 to
-		 either STORE_FLAG_VALUE or zero.  If this is an NE operation,
-		 ORIG_CODE is the actual comparison being done; if it is an EQ,
-		 we must reverse ORIG_CODE.  On machine with a negative value
-		 for STORE_FLAG_VALUE, also look at LT and GE operations.  */
-	      || ((code == NE
-		   || (code == LT
-		       && GET_MODE_CLASS (inner_mode) == MODE_INT
-		       && (GET_MODE_BITSIZE (inner_mode)
-			   <= HOST_BITS_PER_WIDE_INT)
-		       && (STORE_FLAG_VALUE
-			   & ((HOST_WIDE_INT) 1
-			      << (GET_MODE_BITSIZE (inner_mode) - 1))))
+	  /* `(COMPARE A B) != 0)' is equivalent to `(COMPARE A B)'.
+	     If CODE is EQ, rather than NE, then we are out of luck;
+	     there is no way to reverse the sense of a COMPARE.  */
+	  if (code == NE && GET_CODE (p->exp) == COMPARE)
+	    {
+	      x = p->exp;
+	      break;
+	    }
+	  /* Another possibility is that this machine has a compare
+	     insn that includes the comparison code.  In that case,
+	     ARG1 would be equivalent to a comparison operation that
+	     would set ARG1 to either STORE_FLAG_VALUE or zero.  If
+	     this is an NE operation, ORIG_CODE is the actual
+	     comparison being done; if it is an EQ, we must reverse
+	     ORIG_CODE.  On machine with a negative value for
+	     STORE_FLAG_VALUE, also look at LT and GE operations.  */
+	  else if ((code == NE
+		    || (code == LT
+			&& GET_MODE_CLASS (inner_mode) == MODE_INT
+			&& (GET_MODE_BITSIZE (inner_mode)
+			    <= HOST_BITS_PER_WIDE_INT)
+			&& (STORE_FLAG_VALUE
+			    & ((HOST_WIDE_INT) 1
+			       << (GET_MODE_BITSIZE (inner_mode) - 1))))
 #ifdef FLOAT_STORE_FLAG_VALUE
-		   || (code == LT
-		       && GET_MODE_CLASS (inner_mode) == MODE_FLOAT
-		       && (REAL_VALUE_NEGATIVE
-			   (FLOAT_STORE_FLAG_VALUE (GET_MODE (arg1)))))
+		    || (code == LT
+			&& GET_MODE_CLASS (inner_mode) == MODE_FLOAT
+			&& (REAL_VALUE_NEGATIVE
+			    (FLOAT_STORE_FLAG_VALUE (GET_MODE (arg1)))))
 #endif
-		   )
-		  && GET_RTX_CLASS (GET_CODE (p->exp)) == '<'))
+		    )
+		   && GET_RTX_CLASS (GET_CODE (p->exp)) == '<')
 	    {
 	      x = p->exp;
 	      break;