[Openmp-commits] [PATCH] D97079: [OpenMP] libomp: eliminate pause from atomic CAS loops

[Andrey Churbanov via Phabricator via Openmp-commits]

AndreyChurbanov created this revision.
AndreyChurbanov added reviewers: hbae, jlpeyton, tlwilmar, Nawrin.
AndreyChurbanov added a project: OpenMP.
Herald added subscribers: jfb, guansong, yaxunl.
AndreyChurbanov requested review of this revision.
Herald added a reviewer: jdoerfert.
Herald added subscribers: openmp-commits, sstefan1.

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.

So pause only causes unnecessary slowdown.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97079

Files:
  openmp/runtime/src/kmp_atomic.cpp


Index: openmp/runtime/src/kmp_atomic.cpp
===================================================================
--- openmp/runtime/src/kmp_atomic.cpp
+++ openmp/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;                                                         \


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