[PATCH, i386]: Fix PR91719, emit XCHG for seq_cst store on big cores

[PATCH, i386]: Fix PR91719, emit XCHG for seq_cst store on big cores

[Uros Bizjak]

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Attached patch emits XCHG instead of store+MFENCE on big cores and
generic tuning

m_CORE_ALL | m_BDVER | m_ZNVER | m_GENERIC

(The tune can be added for other targets, too.)

2019-09-16  Uroš Bizjak  <ubizjak@gmail.com>

    PR target/91719
    * config/i386/i386.h (TARGET_USE_XCHG_FOR_ATOMIC_STORE): New macro.
    * config/i386/x86-tune.def (X86_TUNE_USE_XCHG_FOR_ATOMIC_STORE): New.
    * config/i386/sync.md (atomic_store<mode>): emit XCHG for
    TARGET_USE_XCHG_FOR_ATOMIC_STORE.

Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.

Committed to mainline SVN.

Uros.

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Index: config/i386/i386.h
===================================================================
--- config/i386/i386.h	(revision 275752)
+++ config/i386/i386.h	(working copy)
@@ -590,6 +590,8 @@ extern unsigned char ix86_tune_features[X86_TUNE_L
 	ix86_tune_features[X86_TUNE_AVOID_FALSE_DEP_FOR_BMI]
 #define TARGET_ONE_IF_CONV_INSN \
 	ix86_tune_features[X86_TUNE_ONE_IF_CONV_INSN]
+#define TARGET_USE_XCHG_FOR_ATOMIC_STORE \
+	ix86_tune_features[X86_TUNE_USE_XCHG_FOR_ATOMIC_STORE]
 #define TARGET_EMIT_VZEROUPPER \
 	ix86_tune_features[X86_TUNE_EMIT_VZEROUPPER]
 
Index: config/i386/sync.md
===================================================================
--- config/i386/sync.md	(revision 275752)
+++ config/i386/sync.md	(working copy)
@@ -306,8 +306,11 @@
     {
       operands[1] = force_reg (<MODE>mode, operands[1]);
 
-      /* For seq-cst stores, when we lack MFENCE, use XCHG.  */
-      if (is_mm_seq_cst (model) && !(TARGET_64BIT || TARGET_SSE2))
+      /* For seq-cst stores, use XCHG
+	 when we lack MFENCE or when target prefers XCHG.  */
+      if (is_mm_seq_cst (model)
+	  && (!(TARGET_64BIT || TARGET_SSE2)
+	      || TARGET_USE_XCHG_FOR_ATOMIC_STORE))
 	{
 	  emit_insn (gen_atomic_exchange<mode> (gen_reg_rtx (<MODE>mode),
 						operands[0], operands[1],
Index: config/i386/x86-tune.def
===================================================================
--- config/i386/x86-tune.def	(revision 275752)
+++ config/i386/x86-tune.def	(working copy)
@@ -313,6 +313,10 @@ DEF_TUNE (X86_TUNE_ONE_IF_CONV_INSN, "one_if_conv_
 	  m_SILVERMONT | m_KNL | m_KNM | m_INTEL | m_CORE_ALL | m_GOLDMONT
 	  | m_GOLDMONT_PLUS | m_TREMONT | m_GENERIC)
 
+/* X86_TUNE_USE_XCHG_FOR_ATOMIC_STORE: Use xchg instead of mov+mfence.  */
+DEF_TUNE (X86_TUNE_USE_XCHG_FOR_ATOMIC_STORE, "use_xchg_for_atomic_store",
+	 m_CORE_ALL | m_BDVER | m_ZNVER | m_GENERIC)
+
 /*****************************************************************************/
 /* 387 instruction selection tuning                                          */
 /*****************************************************************************/