[PATCH] PR target/103069: Relax cmpxchg loop for x86 target

Hongyu Wang hongyu.wang@intel.com
Sat Nov 13 02:34:09 GMT 2021


Hi,

>From the CPU's point of view, getting a cache line for writing is more
expensive than reading.  See Appendix A.2 Spinlock in:

https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/
xeon-lock-scaling-analysis-paper.pdf

The full compare and swap will grab the cache line exclusive and causes
excessive cache line bouncing.

The atomic_fetch_{or,xor,and,nand} builtins generates cmpxchg loop under
-march=x86-64 like:

	movl	(%rdi), %eax
.L2:
	movl	%eax, %edx
	movl	%eax, %r8d
	orl	$esi, %edx
	lock cmpxchgl	%edx, (%rdi)
	jne	.L2
	movl	%r8d, %eax
	ret

To relax above loop, GCC should first emit a normal load, check and jump to
.L2 if cmpxchgl may fail. Before jump to .L2, PAUSE should be inserted to
yield the CPU to another hyperthread and to save power, so the code is
like

	movl	(%rdi), %eax
.L4:
        movl    (%rdi), %ecx
        movl    %eax, %edx
        orl     %esi, %edx
        cmpl    %eax, %ecx
        jne     .L2
        lock cmpxchgl   %edx, (%rdi)
        jne     .L4
.L2:
        rep nop
        jmp     .L4

This patch adds corresponding atomic_fetch_op expanders to insert load/
compare and pause for all the atomic logic fetch builtins. Add flag
-mrelax-cmpxchg-loop to control whether to generate relaxed loop.

Bootstraped/regtested on x86_64-pc-linux-gnu{-m32,}.
Ok for master?

gcc/ChangeLog:

	PR target/103069
	* config/i386/i386-expand.c (ix86_expand_atomic_fetch_op_loop):
	New expand function.
	* config/i386/i386-options.c (ix86_target_string): Add
	-mrelax-cmpxchg-loop flag.
	(ix86_valid_target_attribute_inner_p): Likewise.
	* config/i386/i386-protos.h (ix86_expand_atomic_fetch_op_loop):
	New expand function prototype.
	* config/i386/i386.opt: Add -mrelax-cmpxchg-loop.
	* config/i386/sync.md (atomic_fetch_<logic><mode>): New expander
	for SI,HI,QI modes.
	(atomic_<logic>_fetch<mode>): Likewise.
	(atomic_fetch_nand<mode>): Likewise.
	(atomic_nand_fetch<mode>): Likewise.
	(atomic_fetch_<logic><mode>): New expander for DI,TI modes.
	(atomic_<logic>_fetch<mode>): Likewise.
	(atomic_fetch_nand<mode>): Likewise.
	(atomic_nand_fetch<mode>): Likewise.
	* doc/invoke.texi: Document -mrelax-cmpxchg-loop.

gcc/testsuite/ChangeLog:

	PR target/103069
	* gcc.target/i386/pr103069-1.c: New test.
	* gcc.target/i386/pr103069-2.c: Ditto.
---
 gcc/config/i386/i386-expand.c              |  77 ++++++++++++++
 gcc/config/i386/i386-options.c             |   7 +-
 gcc/config/i386/i386-protos.h              |   2 +
 gcc/config/i386/i386.opt                   |   4 +
 gcc/config/i386/sync.md                    | 117 +++++++++++++++++++++
 gcc/doc/invoke.texi                        |   9 +-
 gcc/testsuite/gcc.target/i386/pr103069-1.c |  35 ++++++
 gcc/testsuite/gcc.target/i386/pr103069-2.c |  70 ++++++++++++
 8 files changed, 319 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.target/i386/pr103069-1.c
 create mode 100644 gcc/testsuite/gcc.target/i386/pr103069-2.c

diff --git a/gcc/config/i386/i386-expand.c b/gcc/config/i386/i386-expand.c
index 088e6af2258..f8a61835d85 100644
--- a/gcc/config/i386/i386-expand.c
+++ b/gcc/config/i386/i386-expand.c
@@ -23138,4 +23138,81 @@ ix86_expand_divmod_libfunc (rtx libfunc, machine_mode mode,
   *rem_p = rem;
 }
 
+void ix86_expand_atomic_fetch_op_loop (rtx target, rtx mem, rtx val,
+				       enum rtx_code code, bool after,
+				       bool doubleword)
+{
+  rtx old_reg, new_reg, old_mem, success, oldval, new_mem;
+  rtx_code_label *loop_label, *pause_label;
+  machine_mode mode = GET_MODE (target);
+
+  old_reg = gen_reg_rtx (mode);
+  new_reg = old_reg;
+  loop_label = gen_label_rtx ();
+  pause_label = gen_label_rtx ();
+  old_mem = copy_to_reg (mem);
+  emit_label (loop_label);
+  emit_move_insn (old_reg, old_mem);
+
+  /* return value for atomic_fetch_op.  */
+  if (!after)
+    emit_move_insn (target, old_reg);
+
+  if (code == NOT)
+    {
+      new_reg = expand_simple_binop (mode, AND, new_reg, val, NULL_RTX,
+				     true, OPTAB_LIB_WIDEN);
+      new_reg = expand_simple_unop (mode, code, new_reg, NULL_RTX, true);
+    }
+  else
+    new_reg = expand_simple_binop (mode, code, new_reg, val, NULL_RTX,
+				   true, OPTAB_LIB_WIDEN);
+
+  /* return value for atomic_op_fetch.  */
+  if (after)
+    emit_move_insn (target, new_reg);
+
+  /* Load memory again inside loop.  */
+  new_mem = copy_to_reg (mem);
+  /* Compare mem value with expected value.  */
+
+  if (doubleword)
+    {
+      machine_mode half_mode = (mode == DImode)? SImode : DImode;
+      rtx low_new_mem = gen_lowpart (half_mode, new_mem);
+      rtx low_old_mem = gen_lowpart (half_mode, old_mem);
+      rtx high_new_mem = gen_highpart (half_mode, new_mem);
+      rtx high_old_mem = gen_highpart (half_mode, old_mem);
+      emit_cmp_and_jump_insns (low_new_mem, low_old_mem, NE, NULL_RTX,
+			       half_mode, 1, pause_label,
+			       profile_probability::guessed_never ());
+      emit_cmp_and_jump_insns (high_new_mem, high_old_mem, NE, NULL_RTX,
+			       half_mode, 1, pause_label,
+			       profile_probability::guessed_never ());
+    }
+  else
+    emit_cmp_and_jump_insns (new_mem, old_mem, NE, NULL_RTX,
+			     GET_MODE (old_mem), 1, pause_label,
+			     profile_probability::guessed_never ());
+
+  success = NULL_RTX;
+  oldval = old_mem;
+  expand_atomic_compare_and_swap (&success, &oldval, mem, old_reg,
+				  new_reg, false, MEMMODEL_SYNC_SEQ_CST,
+				  MEMMODEL_RELAXED);
+  if (oldval != old_mem)
+    emit_move_insn (old_mem, oldval);
+
+  emit_cmp_and_jump_insns (success, const0_rtx, EQ, const0_rtx,
+			   GET_MODE (success), 1, loop_label,
+			   profile_probability::guessed_never ());
+
+  /* If mem is not expected, pause and loop back.  */
+  emit_label (pause_label);
+  emit_insn (gen_pause ());
+  emit_jump_insn (gen_jump (loop_label));
+  emit_barrier ();
+}
+
+
 #include "gt-i386-expand.h"
diff --git a/gcc/config/i386/i386-options.c b/gcc/config/i386/i386-options.c
index a8cc0664f11..feff2584f41 100644
--- a/gcc/config/i386/i386-options.c
+++ b/gcc/config/i386/i386-options.c
@@ -397,7 +397,8 @@ ix86_target_string (HOST_WIDE_INT isa, HOST_WIDE_INT isa2,
     { "-mstv",				MASK_STV },
     { "-mavx256-split-unaligned-load",	MASK_AVX256_SPLIT_UNALIGNED_LOAD },
     { "-mavx256-split-unaligned-store",	MASK_AVX256_SPLIT_UNALIGNED_STORE },
-    { "-mcall-ms2sysv-xlogues",		MASK_CALL_MS2SYSV_XLOGUES }
+    { "-mcall-ms2sysv-xlogues",		MASK_CALL_MS2SYSV_XLOGUES },
+    { "-mrelax-cmpxchg-loop",		MASK_RELAX_CMPXCHG_LOOP }
   };
 
   /* Additional flag options.  */
@@ -1092,6 +1093,10 @@ ix86_valid_target_attribute_inner_p (tree fndecl, tree args, char *p_strings[],
     IX86_ATTR_IX86_YES ("general-regs-only",
 			OPT_mgeneral_regs_only,
 			OPTION_MASK_GENERAL_REGS_ONLY),
+
+    IX86_ATTR_YES ("relax-cmpxchg-loop",
+		   OPT_mrelax_cmpxchg_loop,
+		   MASK_RELAX_CMPXCHG_LOOP),
   };
 
   location_t loc
diff --git a/gcc/config/i386/i386-protos.h b/gcc/config/i386/i386-protos.h
index bd52450a148..7e05510c679 100644
--- a/gcc/config/i386/i386-protos.h
+++ b/gcc/config/i386/i386-protos.h
@@ -217,6 +217,8 @@ extern void ix86_move_vector_high_sse_to_mmx (rtx);
 extern void ix86_split_mmx_pack (rtx[], enum rtx_code);
 extern void ix86_split_mmx_punpck (rtx[], bool);
 extern void ix86_expand_avx_vzeroupper (void);
+extern void ix86_expand_atomic_fetch_op_loop (rtx, rtx, rtx, enum rtx_code,
+					      bool, bool);
 
 #ifdef TREE_CODE
 extern void init_cumulative_args (CUMULATIVE_ARGS *, tree, rtx, tree, int);
diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt
index ad366974b5b..46fad3cc038 100644
--- a/gcc/config/i386/i386.opt
+++ b/gcc/config/i386/i386.opt
@@ -404,6 +404,10 @@ momit-leaf-frame-pointer
 Target Mask(OMIT_LEAF_FRAME_POINTER) Save
 Omit the frame pointer in leaf functions.
 
+mrelax-cmpxchg-loop
+Target Mask(RELAX_CMPXCHG_LOOP) Save
+Relax cmpxchg loop for atomic_fetch_{or,xor,and,nand} by adding load and cmp before cmpxchg, execute pause and loop back to load and compare if load value is not expected.
+
 mpc32
 Target RejectNegative
 Set 80387 floating-point precision to 32-bit.
diff --git a/gcc/config/i386/sync.md b/gcc/config/i386/sync.md
index 05a835256bb..46048425327 100644
--- a/gcc/config/i386/sync.md
+++ b/gcc/config/i386/sync.md
@@ -525,6 +525,123 @@
 	      (set (reg:CCZ FLAGS_REG)
 		   (unspec_volatile:CCZ [(const_int 0)] UNSPECV_CMPXCHG))])])
 
+(define_expand "atomic_fetch_<logic><mode>"
+  [(match_operand:SWI124 0 "register_operand")
+   (any_logic:SWI124
+    (match_operand:SWI124 1 "memory_operand")
+    (match_operand:SWI124 2 "register_operand"))
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], <CODE>, false,
+				    false);
+  DONE;
+})
+
+(define_expand "atomic_<logic>_fetch<mode>"
+  [(match_operand:SWI124 0 "register_operand")
+   (any_logic:SWI124
+    (match_operand:SWI124 1 "memory_operand")
+    (match_operand:SWI124 2 "register_operand"))
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], <CODE>, true,
+				    false);
+  DONE;
+})
+
+(define_expand "atomic_fetch_nand<mode>"
+  [(match_operand:SWI124 0 "register_operand")
+   (match_operand:SWI124 1 "memory_operand")
+   (match_operand:SWI124 2 "register_operand")
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], NOT, false,
+				    false);
+  DONE;
+})
+
+(define_expand "atomic_nand_fetch<mode>"
+  [(match_operand:SWI124 0 "register_operand")
+   (match_operand:SWI124 1 "memory_operand")
+   (match_operand:SWI124 2 "register_operand")
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], NOT, true,
+				    false);
+  DONE;
+})
+
+(define_expand "atomic_fetch_<logic><mode>"
+  [(match_operand:CASMODE 0 "register_operand")
+   (any_logic:CASMODE
+    (match_operand:CASMODE 1 "memory_operand")
+    (match_operand:CASMODE 2 "register_operand"))
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  bool doubleword = (<MODE>mode == DImode && !TARGET_64BIT)
+		    || (<MODE>mode == TImode);
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], <CODE>, false,
+				    doubleword);
+  DONE;
+})
+
+(define_expand "atomic_<logic>_fetch<mode>"
+  [(match_operand:CASMODE 0 "register_operand")
+   (any_logic:CASMODE
+    (match_operand:CASMODE 1 "memory_operand")
+    (match_operand:CASMODE 2 "register_operand"))
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  bool doubleword = (<MODE>mode == DImode && !TARGET_64BIT)
+		    || (<MODE>mode == TImode);
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], <CODE>, true,
+				    doubleword);
+  DONE;
+})
+
+(define_expand "atomic_fetch_nand<mode>"
+  [(match_operand:CASMODE 0 "register_operand")
+   (match_operand:CASMODE 1 "memory_operand")
+   (match_operand:CASMODE 2 "register_operand")
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  bool doubleword = (<MODE>mode == DImode && !TARGET_64BIT)
+		    || (<MODE>mode == TImode);
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], NOT, false,
+				    doubleword);
+  DONE;
+})
+
+(define_expand "atomic_nand_fetch<mode>"
+  [(match_operand:CASMODE 0 "register_operand")
+   (match_operand:CASMODE 1 "memory_operand")
+   (match_operand:CASMODE 2 "register_operand")
+   (match_operand:SI 3 "const_int_operand")]
+  "TARGET_CMPXCHG && TARGET_RELAX_CMPXCHG_LOOP"
+{
+  bool doubleword = (<MODE>mode == DImode && !TARGET_64BIT)
+		    || (<MODE>mode == TImode);
+  ix86_expand_atomic_fetch_op_loop (operands[0], operands[1],
+				    operands[2], NOT, true,
+				    doubleword);
+  DONE;
+})
+
+
 ;; For operand 2 nonmemory_operand predicate is used instead of
 ;; register_operand to allow combiner to better optimize atomic
 ;; additions of constants.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 2aba4c70b44..06ecf79bc0c 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -1419,7 +1419,7 @@ See RS/6000 and PowerPC Options.
 -mstack-protector-guard-reg=@var{reg} @gol
 -mstack-protector-guard-offset=@var{offset} @gol
 -mstack-protector-guard-symbol=@var{symbol} @gol
--mgeneral-regs-only  -mcall-ms2sysv-xlogues @gol
+-mgeneral-regs-only  -mcall-ms2sysv-xlogues -mrelax-cmpxchg-loop @gol
 -mindirect-branch=@var{choice}  -mfunction-return=@var{choice} @gol
 -mindirect-branch-register -mneeded}
 
@@ -32259,6 +32259,13 @@ Generate code that uses only the general-purpose registers.  This
 prevents the compiler from using floating-point, vector, mask and bound
 registers.
 
+@item -mrelax-cmpxchg-loop
+@opindex mrelax-cmpxchg-loop
+Relax cmpxchg loop by emit early load and compare before cmpxchg,
+execute pause if load value is not expected. This reduces excessive
+cachline bouncing when and works for all atomic logic fetch builtins
+that generates compare and swap loop.
+
 @item -mindirect-branch=@var{choice}
 @opindex mindirect-branch
 Convert indirect call and jump with @var{choice}.  The default is
diff --git a/gcc/testsuite/gcc.target/i386/pr103069-1.c b/gcc/testsuite/gcc.target/i386/pr103069-1.c
new file mode 100644
index 00000000000..444485cbae9
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr103069-1.c
@@ -0,0 +1,35 @@
+/* PR target/103068 */
+/* { dg-do compile } */
+/* { dg-additional-options "-O2 -march=x86-64 -mtune=generic -mrelax-cmpxchg-loop" } */ 
+/* { dg-final { scan-assembler-times "rep nop" 32 } } */
+
+#include <stdint.h>
+
+#define FUNC_ATOMIC(TYPE, OP) \
+__attribute__ ((noinline, noclone))	\
+TYPE f_##TYPE##_##OP##_fetch (TYPE *a, TYPE b)	\
+{ \
+  return __atomic_##OP##_fetch (a, b, __ATOMIC_RELAXED);  \
+} \
+__attribute__ ((noinline, noclone))	\
+TYPE f_##TYPE##_fetch_##OP (TYPE *a, TYPE b)	\
+{ \
+  return __atomic_fetch_##OP (a, b, __ATOMIC_RELAXED);  \
+}
+
+FUNC_ATOMIC (int64_t, and)
+FUNC_ATOMIC (int64_t, nand)
+FUNC_ATOMIC (int64_t, or)
+FUNC_ATOMIC (int64_t, xor)
+FUNC_ATOMIC (int, and)
+FUNC_ATOMIC (int, nand)
+FUNC_ATOMIC (int, or)
+FUNC_ATOMIC (int, xor)
+FUNC_ATOMIC (short, and)
+FUNC_ATOMIC (short, nand)
+FUNC_ATOMIC (short, or)
+FUNC_ATOMIC (short, xor)
+FUNC_ATOMIC (char, and)
+FUNC_ATOMIC (char, nand)
+FUNC_ATOMIC (char, or)
+FUNC_ATOMIC (char, xor)
diff --git a/gcc/testsuite/gcc.target/i386/pr103069-2.c b/gcc/testsuite/gcc.target/i386/pr103069-2.c
new file mode 100644
index 00000000000..8ac824cc8e8
--- /dev/null
+++ b/gcc/testsuite/gcc.target/i386/pr103069-2.c
@@ -0,0 +1,70 @@
+/* PR target/103068 */
+/* { dg-do compile } */
+/* { dg-additional-options "-O2 -march=x86-64 -mtune=generic" } */ 
+
+#include <stdlib.h>
+#include "pr103069-1.c"
+
+#define FUNC_ATOMIC_RELAX(TYPE, OP) \
+__attribute__ ((noinline, noclone, target ("relax-cmpxchg-loop")))	\
+TYPE relax_##TYPE##_##OP##_fetch (TYPE *a, TYPE b)	\
+{ \
+  return __atomic_##OP##_fetch (a, b, __ATOMIC_RELAXED);  \
+} \
+__attribute__ ((noinline, noclone, target ("relax-cmpxchg-loop")))	\
+TYPE relax_##TYPE##_fetch_##OP (TYPE *a, TYPE b)	\
+{ \
+  return __atomic_fetch_##OP (a, b, __ATOMIC_RELAXED);  \
+}
+
+FUNC_ATOMIC_RELAX (int64_t, and)
+FUNC_ATOMIC_RELAX (int64_t, nand)
+FUNC_ATOMIC_RELAX (int64_t, or)
+FUNC_ATOMIC_RELAX (int64_t, xor)
+FUNC_ATOMIC_RELAX (int, and)
+FUNC_ATOMIC_RELAX (int, nand)
+FUNC_ATOMIC_RELAX (int, or)
+FUNC_ATOMIC_RELAX (int, xor)
+FUNC_ATOMIC_RELAX (short, and)
+FUNC_ATOMIC_RELAX (short, nand)
+FUNC_ATOMIC_RELAX (short, or)
+FUNC_ATOMIC_RELAX (short, xor)
+FUNC_ATOMIC_RELAX (char, and)
+FUNC_ATOMIC_RELAX (char, nand)
+FUNC_ATOMIC_RELAX (char, or)
+FUNC_ATOMIC_RELAX (char, xor)
+
+#define TEST_ATOMIC_FETCH_LOGIC(TYPE, OP) \
+{ \
+  TYPE a = 11, b = 101, res, exp; \
+  res = relax_##TYPE##_##OP##_fetch (&a, b); \
+  exp = f_##TYPE##_##OP##_fetch (&a, b);  \
+  if (res != exp) \
+    abort (); \
+  a = 21, b = 92; \
+  res = relax_##TYPE##_fetch_##OP (&a, b); \
+  exp = f_##TYPE##_fetch_##OP (&a, b);  \
+  if (res != exp) \
+    abort (); \
+}
+
+int main (void)
+{
+  TEST_ATOMIC_FETCH_LOGIC (int64_t, and)
+  TEST_ATOMIC_FETCH_LOGIC (int64_t, nand)
+  TEST_ATOMIC_FETCH_LOGIC (int64_t, or)
+  TEST_ATOMIC_FETCH_LOGIC (int64_t, xor)
+  TEST_ATOMIC_FETCH_LOGIC (int, and)
+  TEST_ATOMIC_FETCH_LOGIC (int, nand)
+  TEST_ATOMIC_FETCH_LOGIC (int, or)
+  TEST_ATOMIC_FETCH_LOGIC (int, xor)
+  TEST_ATOMIC_FETCH_LOGIC (short, and)
+  TEST_ATOMIC_FETCH_LOGIC (short, nand)
+  TEST_ATOMIC_FETCH_LOGIC (short, or)
+  TEST_ATOMIC_FETCH_LOGIC (short, xor)
+  TEST_ATOMIC_FETCH_LOGIC (char, and)
+  TEST_ATOMIC_FETCH_LOGIC (char, nand)
+  TEST_ATOMIC_FETCH_LOGIC (char, or)
+  TEST_ATOMIC_FETCH_LOGIC (char, xor)
+  return 0;
+}
-- 
2.18.1



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