This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
[Patch, i386] Avoid LCP stalls (issue5975045)
- From: tejohnson at google dot com (Teresa Johnson)
- To: reply at codereview dot appspotmail dot com,gcc-patches at gcc dot gnu dot org
- Date: Fri, 30 Mar 2012 07:18:58 -0700 (PDT)
- Subject: [Patch, i386] Avoid LCP stalls (issue5975045)
This patch addresses 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. The attached patch avoids this by forcing such instructions to be split into
two: a move of the corresponding 32-bit constant into a register, and a move of the
register's lower 16 bits into memory.
Bootstrapped and tested on x86_64-unknown-linux-gnu. Is this ok for trunk?
Thanks,
Teresa
2012-03-29 Teresa Johnson <tejohnson@google.com>
* config/i386/i386.h (ix86_tune_indices): Add
X86_TUNE_LCP_STALL.
* config/i386/i386.md (movhi_internal): Split to
movhi_internal and movhi_imm_internal.
* config/i386/i386.c (initial_ix86_tune_features): Initialize
X86_TUNE_LCP_STALL entry.
Index: config/i386/i386.h
===================================================================
--- config/i386/i386.h (revision 185920)
+++ config/i386/i386.h (working copy)
@@ -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_L
#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]
Index: config/i386/i386.md
===================================================================
--- config/i386/i386.md (revision 185920)
+++ config/i386/i386.md (working copy)
@@ -2262,9 +2262,19 @@
]
(const_string "SI")))])
+(define_insn "*movhi_imm_internal"
+ [(set (match_operand:HI 0 "memory_operand" "=m")
+ (match_operand:HI 1 "immediate_operand" "n"))]
+ "!TARGET_LCP_STALL && !(MEM_P (operands[0]) && MEM_P (operands[1]))"
+{
+ return "mov{w}\t{%1, %0|%0, %1}";
+}
+ [(set (attr "type") (const_string "imov"))
+ (set (attr "mode") (const_string "HI"))])
+
(define_insn "*movhi_internal"
[(set (match_operand:HI 0 "nonimmediate_operand" "=r,r,r,m")
- (match_operand:HI 1 "general_operand" "r,rn,rm,rn"))]
+ (match_operand:HI 1 "general_operand" "r,rn,rm,r"))]
"!(MEM_P (operands[0]) && MEM_P (operands[1]))"
{
switch (get_attr_type (insn))
Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c (revision 185920)
+++ config/i386/i386.c (working copy)
@@ -1964,6 +1964,11 @@ static unsigned int initial_ix86_tune_features[X86
/* 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,
+ * which may also affect AMD implementations. */
+ m_CORE2I7 | m_GENERIC | m_AMD_MULTIPLE,
+
/* X86_TUNE_USE_HIMODE_FIOP */
m_386 | m_486 | m_K6_GEODE,
--
This patch is available for review at http://codereview.appspot.com/5975045