[Profile][Windows] Fix flakyness when checking existence of binary id

[ZequanWu]

There is a small chance that binary id starting with 0 (1/256). It is not sufficient to just check the first byte.

[llvmbot]

@llvm/pr-subscribers-pgo

Author: Zequan Wu (ZequanWu)

Changes

There is a small chance that binary id starting with 0 (1/256). It is not sufficient to just check the first byte.

---

Full diff: https://github.com/llvm/llvm-project/pull/84196.diff

1 Files Affected:

• (modified) compiler-rt/lib/profile/InstrProfilingPlatformWindows.c (+3-2)

diff --git a/compiler-rt/lib/profile/InstrProfilingPlatformWindows.c b/compiler-rt/lib/profile/InstrProfilingPlatformWindows.c
index b9642ca7f6810f..f2bb02efa50078 100644
--- a/compiler-rt/lib/profile/InstrProfilingPlatformWindows.c
+++ b/compiler-rt/lib/profile/InstrProfilingPlatformWindows.c
@@ -96,9 +96,10 @@ ValueProfNode *EndVNode = &VNodesEnd;
 /* lld-link provides __buildid symbol which ponits to the 16 bytes build id when
  * using /build-id flag. https://lld.llvm.org/windows_support.html#lld-flags */
 #define BUILD_ID_LEN 16
-COMPILER_RT_WEAK uint8_t __buildid[BUILD_ID_LEN];
+COMPILER_RT_WEAK uint8_t __buildid[BUILD_ID_LEN] = {0};
 COMPILER_RT_VISIBILITY int __llvm_write_binary_ids(ProfDataWriter *Writer) {
-  if (*__buildid) {
+  uint8_t zeros[BUILD_ID_LEN] = {0};
+  if (memcmp(__buildid, zeros, BUILD_ID_LEN) != 0) {
     if (Writer &&
         lprofWriteOneBinaryId(Writer, BUILD_ID_LEN, __buildid, 0) == -1)
       return -1;

[zmodem]

lgtm with nit

Can a build-id be all-zeros? If yes, maybe as a followup we should ensure that it can't.

[ZequanWu]

lgtm with nit

Can a build-id be all-zeros? If yes, maybe as a followup we should ensure that it can't.

build-id provided by lld-link is generated by the hash of the object file or pdb: https://github.com/llvm/llvm-project/blob/main/lld/COFF/Writer.cpp#L2171, which is almost impossible to be all 0s.

[zmodem]

which is almost impossible to be all 0s.

It may be unlikely, but it sounds like it's definitely possible.
If all-zero build-ids are not supported, maybe that code should do something like if (hash == 0) ++hash;