Avoid instructions that incur expensive length-changing prefix (LCP) stalls on some x86-64 implementations...

Avoid instructions that incur expensive length-changing prefix (LCP) stalls
on some x86-64 implementations, notably Core2 and Corei7. Specifically, a move of
a 16-bit constant into memory requires a length-changing prefix and can incur significant
penalties. Modified an old patch written by H.J to split such instructions
during peephole2.

2012-04-05  Teresa Johnson  <tejohnson@google.com>
	    H.J. Lu  <hongjiu.lu@intel.com>

	* config/i386/i386.h (ix86_tune_indices): Add
	X86_TUNE_LCP_STALL.
	* config/i386/i386.md (move immediate to memory peephole2):
	Add cases for HImode move when LCP stall avoidance is needed.
	* config/i386/i386.c (initial_ix86_tune_features): Initialize
	X86_TUNE_LCP_STALL entry.

Co-Authored-By: H.J. Lu <hongjiu.lu@intel.com>

From-SVN: r186176

4 files changed, 27 insertions, 6 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 0df25cf..8118ed5 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,13 @@
+2012-04-05  Teresa Johnson  <tejohnson@google.com>
+	    H.J. Lu  <hongjiu.lu@intel.com>
+
+	* config/i386/i386.h (ix86_tune_indices): Add
+	X86_TUNE_LCP_STALL.
+	* config/i386/i386.md (move immediate to memory peephole2):
+	Add cases for HImode move when LCP stall avoidance is needed.
+	* config/i386/i386.c (initial_ix86_tune_features): Initialize
+	X86_TUNE_LCP_STALL entry.
+
 2012-04-05  Uros Bizjak  <ubizjak@gmail.com>
 
 	PR target/52882
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index c959113..8974ddc 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -1964,6 +1964,10 @@ static unsigned int initial_ix86_tune_features[X86_TUNE_LAST] = {
   /* X86_TUNE_PARTIAL_FLAG_REG_STALL */
   m_CORE2I7 | m_GENERIC,
 
+  /* X86_TUNE_LCP_STALL: Avoid an expensive length-changing prefix stall
+   * on 16-bit immediate moves into memory on Core2 and Corei7.  */
+  m_CORE2I7 | m_GENERIC,
+
   /* X86_TUNE_USE_HIMODE_FIOP */
   m_386 | m_486 | m_K6_GEODE,
 
diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h
index 7ba90c7..8942ea8 100644
--- a/gcc/config/i386/i386.h
+++ b/gcc/config/i386/i386.h
@@ -262,6 +262,7 @@ enum ix86_tune_indices {
   X86_TUNE_MOVX,
   X86_TUNE_PARTIAL_REG_STALL,
   X86_TUNE_PARTIAL_FLAG_REG_STALL,
+  X86_TUNE_LCP_STALL,
   X86_TUNE_USE_HIMODE_FIOP,
   X86_TUNE_USE_SIMODE_FIOP,
   X86_TUNE_USE_MOV0,
@@ -340,6 +341,8 @@ extern unsigned char ix86_tune_features[X86_TUNE_LAST];
 #define TARGET_PARTIAL_REG_STALL ix86_tune_features[X86_TUNE_PARTIAL_REG_STALL]
 #define TARGET_PARTIAL_FLAG_REG_STALL \
 	ix86_tune_features[X86_TUNE_PARTIAL_FLAG_REG_STALL]
+#define TARGET_LCP_STALL \
+	ix86_tune_features[X86_TUNE_LCP_STALL]
 #define TARGET_USE_HIMODE_FIOP	ix86_tune_features[X86_TUNE_USE_HIMODE_FIOP]
 #define TARGET_USE_SIMODE_FIOP	ix86_tune_features[X86_TUNE_USE_SIMODE_FIOP]
 #define TARGET_USE_MOV0		ix86_tune_features[X86_TUNE_USE_MOV0]
diff --git a/gcc/config/i386/i386.md b/gcc/config/i386/i386.md
index 2d20a52..dd1f9be 100644
--- a/gcc/config/i386/i386.md
+++ b/gcc/config/i386/i386.md
@@ -16971,15 +16971,17 @@
    (set (match_dup 0) (match_dup 2))])
 
 ;; Don't move an immediate directly to memory when the instruction
-;; gets too big.
+;; gets too big, or if LCP stalls are a problem for 16-bit moves.
 (define_peephole2
   [(match_scratch:SWI124 1 "<r>")
    (set (match_operand:SWI124 0 "memory_operand")
         (const_int 0))]
   "optimize_insn_for_speed_p ()
-   && !TARGET_USE_MOV0
-   && TARGET_SPLIT_LONG_MOVES
-   && get_attr_length (insn) >= ix86_cur_cost ()->large_insn
+   && ((<MODE>mode == HImode
+       && TARGET_LCP_STALL)
+       || (!TARGET_USE_MOV0
+          && TARGET_SPLIT_LONG_MOVES
+          && get_attr_length (insn) >= ix86_cur_cost ()->large_insn))
    && peep2_regno_dead_p (0, FLAGS_REG)"
   [(parallel [(set (match_dup 2) (const_int 0))
 	      (clobber (reg:CC FLAGS_REG))])
@@ -16991,8 +16993,10 @@
    (set (match_operand:SWI124 0 "memory_operand")
         (match_operand:SWI124 1 "immediate_operand"))]
   "optimize_insn_for_speed_p ()
-   && TARGET_SPLIT_LONG_MOVES
-   && get_attr_length (insn) >= ix86_cur_cost ()->large_insn"
+   && ((<MODE>mode == HImode
+       && TARGET_LCP_STALL)
+       || (TARGET_SPLIT_LONG_MOVES
+          && get_attr_length (insn) >= ix86_cur_cost ()->large_insn))"
   [(set (match_dup 2) (match_dup 1))
    (set (match_dup 0) (match_dup 2))])