commit ea30c8dcea00e7b126bc3cd6cbb5097cd6aa75cc
author AndreyChurbanov <andrey.churbanov@intel.com> Tue Mar 09 18:30:08 2021 +0300
committer Copybara-Service <copybara-worker@google.com> Thu Mar 18 05:45:26 2021 -0700
tree 20e06e61e45b373a10130f707698a7a6641c8e07
parent 31018a5345360ff9375fc00f36d11dc517edcd76

[OpenMP] libomp: eliminate pause from atomic CAS loops

For clang this change is NFC cleanup, because clang
never calls atomic functions from runtime library.

Basically, pause is good in spin-loops waiting for something.
Atomic CAS loops do not wait for anything,
each CAS failure means some other thread progressed.

Performance experiments show that the pause only causes unnecessary slowdown
on CPUs with slow pause instruction, no difference on CPUs with fast pause
instruction, removal of the pause gives lesser binary size which is good.

Differential Revision: https://reviews.llvm.org/D97079

GitOrigin-RevId: aaf16b80dd4cdeb84caae8f3785ce7e4cc7a0f69

runtime/src/kmp_atomic.cpp

diff --git a/runtime/src/kmp_atomic.cpp b/runtime/src/kmp_atomic.cpp
index d219eee..a71a1b3 100644
--- a/runtime/src/kmp_atomic.cpp
+++ b/runtime/src/kmp_atomic.cpp
@@ -779,7 +779,7 @@
 #if KMP_MIC
 #define KMP_DO_PAUSE _mm_delay_32(1)
 #else
-#define KMP_DO_PAUSE KMP_CPU_PAUSE()
+#define KMP_DO_PAUSE
 #endif /* KMP_MIC */
 
 // ------------------------------------------------------------------------
@@ -1132,7 +1132,6 @@
                (kmp_int##BITS *)lhs,                                           \
                *VOLATILE_CAST(kmp_int##BITS *) & old_value,                    \
                *VOLATILE_CAST(kmp_int##BITS *) & rhs)) {                       \
-      KMP_CPU_PAUSE();                                                         \
       temp_val = *lhs;                                                         \
       old_value = temp_val;                                                    \
     }                                                                          \
@@ -2087,8 +2086,6 @@
     while (!KMP_COMPARE_AND_STORE_ACQ##BITS(                                   \
         (kmp_int##BITS *)lhs, *VOLATILE_CAST(kmp_int##BITS *) & old_value,     \
         *VOLATILE_CAST(kmp_int##BITS *) & new_value)) {                        \
-      KMP_CPU_PAUSE();                                                         \
-                                                                               \
       temp_val = *lhs;                                                         \
       old_value = temp_val;                                                    \
       new_value = rhs;                                                         \
@@ -2237,8 +2234,6 @@
     while (!KMP_COMPARE_AND_STORE_ACQ##BITS(                                   \
         (kmp_int##BITS *)lhs, *VOLATILE_CAST(kmp_int##BITS *) & old_value,     \
         *VOLATILE_CAST(kmp_int##BITS *) & new_value)) {                        \
-      KMP_CPU_PAUSE();                                                         \
-                                                                               \
       temp_val = *lhs;                                                         \
       old_value = temp_val;                                                    \
       new_value = (TYPE)(old_value OP rhs);                                    \
@@ -2633,7 +2628,6 @@
                (kmp_int##BITS *)lhs,                                           \
                *VOLATILE_CAST(kmp_int##BITS *) & old_value,                    \
                *VOLATILE_CAST(kmp_int##BITS *) & rhs)) {                       \
-      KMP_CPU_PAUSE();                                                         \
       temp_val = *lhs;                                                         \
       old_value = temp_val;                                                    \
     }                                                                          \
@@ -2930,8 +2924,6 @@
     while (!KMP_COMPARE_AND_STORE_ACQ##BITS(                                   \
         (kmp_int##BITS *)lhs, *VOLATILE_CAST(kmp_int##BITS *) & old_value,     \
         *VOLATILE_CAST(kmp_int##BITS *) & new_value)) {                        \
-      KMP_CPU_PAUSE();                                                         \
-                                                                               \
       temp_val = *lhs;                                                         \
       old_value = temp_val;                                                    \
       new_value = (TYPE)(rhs OP old_value);                                    \
@@ -3254,8 +3246,6 @@
     while (!KMP_COMPARE_AND_STORE_ACQ##BITS(                                   \
         (kmp_int##BITS *)lhs, *VOLATILE_CAST(kmp_int##BITS *) & old_value,     \
         *VOLATILE_CAST(kmp_int##BITS *) & new_value)) {                        \
-      KMP_CPU_PAUSE();                                                         \
-                                                                               \
       temp_val = *lhs;                                                         \
       old_value = temp_val;                                                    \
       new_value = rhs;                                                         \