PR target/103069: Relax cmpxchg loop for x86 target
authorHongyu Wang <hongyu.wang@intel.com>
Fri, 12 Nov 2021 02:50:46 +0000 (10:50 +0800)
committerHongyu Wang <hongyu.wang@intel.com>
Mon, 15 Nov 2021 11:09:38 +0000 (19:09 +0800)
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 v(%rip), %eax
.L2:
movl %eax, %ecx
movl %eax, %edx
orl $1, %ecx
lock cmpxchgl %ecx, v(%rip)
jne .L2
movl %edx, %eax
andl $1, %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

.L84:
        movl    (%rdi), %ecx
        movl    %eax, %edx
        orl     %esi, %edx
        cmpl    %eax, %ecx
        jne     .L82
        lock cmpxchgl   %edx, (%rdi)
        jne     .L84
.L82:
        rep nop
        jmp     .L84

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.

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
gcc/config/i386/i386-options.c
gcc/config/i386/i386-protos.h
gcc/config/i386/i386.opt
gcc/config/i386/sync.md
gcc/doc/invoke.texi
gcc/testsuite/gcc.target/i386/pr103069-1.c [new file with mode: 0644]
gcc/testsuite/gcc.target/i386/pr103069-2.c [new file with mode: 0644]

index 088e6af..3e4de64 100644 (file)
@@ -23138,4 +23138,80 @@ 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"
index a8cc066..feff258 100644 (file)
@@ -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
index bd52450..7e05510 100644 (file)
@@ -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);
index ad36697..46fad3c 100644 (file)
@@ -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.
index 9716a0b..cc4fe72 100644 (file)
              (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.
index 2d9c178..6070288 100644 (file)
@@ -1423,7 +1423,7 @@ 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}
 
@@ -32330,6 +32330,13 @@ 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 emitting an 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 (file)
index 0000000..f819af4
--- /dev/null
@@ -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;?\[ \\t\]+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 (file)
index 0000000..8ac824c
--- /dev/null
@@ -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;
+}