[PATCH] D104349: [compiler-rt][atomics][RFC] Enable __atomic_exchange handling case when `val == old`
Kai Luo via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Tue Jun 15 19:50:37 PDT 2021
lkail created this revision.
lkail added reviewers: jfb, theraven, ldionne, phosek, jsji.
Herald added a subscriber: dberris.
lkail requested review of this revision.
Herald added a project: Sanitizers.
Herald added a subscriber: Sanitizers.
Looks the behavior is not documented when `val == old` in `__atomic_exchange`, so I added code to handle this case.
Or shall we document it as undefined behavior if `val == old`, since when `val == old` and lock code is called, content in `ptr` won't be exchanged finally.
Repository:
rG LLVM Github Monorepo
https://reviews.llvm.org/D104349
Files:
compiler-rt/lib/builtins/atomic.c
Index: compiler-rt/lib/builtins/atomic.c
===================================================================
--- compiler-rt/lib/builtins/atomic.c
+++ compiler-rt/lib/builtins/atomic.c
@@ -233,10 +233,19 @@
return;
LOCK_FREE_CASES(ptr);
#undef LOCK_FREE_ACTION
+ int i;
Lock *l = lock_for_pointer(ptr);
lock(l);
- memcpy(old, ptr, size);
- memcpy(ptr, val, size);
+ if (val != old) {
+ memcpy(old, ptr, size);
+ memcpy(ptr, val, size);
+ } else {
+ for (i = 0; i < size; ++i) {
+ *((char *)ptr + i) ^= *((char *)old + i);
+ *((char *)old + i) ^= *((char *)ptr + i);
+ *((char *)ptr + i) ^= *((char *)old + i);
+ }
+ }
unlock(l);
}
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