Re: [PATCH i386]: Enable push/pop in pro/epilogue for modern CPUs

[Andi Kleen <andi@firstfloor.org>]

Andi Kleen <andi@firstfloor.org> writes:
>
>>> >    /* X86_TUNE_FOUR_JUMP_LIMIT: Some CPU cores are not able to predict more
>>> >       than 4 branch instructions in the 16 byte window.  */
>>> > -  m_PPRO | m_P4_NOCONA | m_CORE2I7 | m_ATOM | m_AMD_MULTIPLE | m_GENERIC,
>>> > +  m_PPRO | m_P4_NOCONA | m_ATOM | m_AMD_MULTIPLE | m_GENERIC,
>>
>> This is special passs to handle limitations of AMD's K7/K8/K10 branch prediction.
>> Intel never had similar design, so this flag is pointless.
>
> Actually the Sandy Bridge decoded icache has a limit of 3 jumps per
> 16 byte window.

Actually it's four per 32bytes, sorry.

Here's an old patch I had lying around to optimize for that.

diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 1b871be..9b57316 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -2713,6 +2713,7 @@ ix86_target_string (HOST_WIDE_INT isa, int flags, const char *arch,
     { "-mavx256-split-unaligned-load",	MASK_AVX256_SPLIT_UNALIGNED_LOAD},
     { "-mavx256-split-unaligned-store",	MASK_AVX256_SPLIT_UNALIGNED_STORE},
     { "-mprefer-avx128",		MASK_PREFER_AVX128},
+    { "-mjump-pad-32bytes",		MASK_JUMP_PAD_32BYTES},
   };
 
   const char *opts[ARRAY_SIZE (isa_opts) + ARRAY_SIZE (flag_opts) + 6][2];
@@ -32182,6 +32183,7 @@ ix86_avoid_jump_mispredicts (void)
   rtx insn, start = get_insns ();
   int nbytes = 0, njumps = 0;
   int isjump = 0;
+  int jump_pad_window_size = TARGET_JUMP_PAD_32BYTES ? 32 : 16;
 
   /* Look for all minimal intervals of instructions containing 4 jumps.
      The intervals are bounded by START and INSN.  NBYTES is the total
@@ -32202,8 +32204,8 @@ ix86_avoid_jump_mispredicts (void)
 	  int align = label_to_alignment (insn);
 	  int max_skip = label_to_max_skip (insn);
 
-	  if (max_skip > 15)
-	    max_skip = 15;
+	  if (max_skip > jump_pad_window_size - 1)
+	    max_skip = jump_pad_window_size - 1;
 	  /* If align > 3, only up to 16 - max_skip - 1 bytes can be
 	     already in the current 16 byte page, because otherwise
 	     ASM_OUTPUT_MAX_SKIP_ALIGN could skip max_skip or fewer
@@ -32216,7 +32218,7 @@ ix86_avoid_jump_mispredicts (void)
 		     INSN_UID (insn), max_skip);
 	  if (max_skip)
 	    {
-	      while (nbytes + max_skip >= 16)
+	      while (nbytes + max_skip >= jump_pad_window_size)
 		{
 		  start = NEXT_INSN (start);
 		  if ((JUMP_P (start)
@@ -32262,10 +32264,11 @@ ix86_avoid_jump_mispredicts (void)
         fprintf (dump_file, "Interval %i to %i has %i bytes\n",
 		 INSN_UID (start), INSN_UID (insn), nbytes);
 
-      if (njumps == 3 && isjump && nbytes < 16)
+      if (njumps == 3 && isjump && nbytes < jump_pad_window_size)
 	{
-	  int padsize = 15 - nbytes + min_insn_size (insn);
-
+	  int padsize = jump_pad_window_size - 1 - nbytes + 
+	    min_insn_size (insn);
+	  
 	  if (dump_file)
 	    fprintf (dump_file, "Padding insn %i by %i bytes!\n",
 		     INSN_UID (insn), padsize);
diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
index 6c516e7..b38d163 100644
--- a/gcc/config/i386/i386.opt
+++ b/gcc/config/i386/i386.opt
@@ -223,6 +223,10 @@ mintel-syntax
 Target Undocumented Alias(masm=, intel, att) Warn(%<-mintel-syntax%> and %<-mno-intel-syntax%> are deprecated; use %<-masm=intel%> and %<-masm=att%> instead)
 ;; Deprecated
 
+mjump-pad-32bytes
+Target RejectNegative Mask(JUMP_PAD_32BYTES) Save
+Avoid more than 4 jumps in each 32byte code window.
+
 mms-bitfields
 Target Report Mask(MS_BITFIELD_LAYOUT) Save
 Use native (MS) bitfield layout


-- 
ak@linux.intel.com -- Speaking for myself only