release/22.x: [MC] Explicitly use memcpy in emitBytes() (NFC) (#177187)#177320
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c-rhodes merged 1 commit intollvm:release/22.xfrom Jan 23, 2026
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release/22.x: [MC] Explicitly use memcpy in emitBytes() (NFC) (#177187)#177320c-rhodes merged 1 commit intollvm:release/22.xfrom
c-rhodes merged 1 commit intollvm:release/22.xfrom
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@MaskRay What do you think about merging this PR to the release branch? |
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@llvm/pr-subscribers-llvm-mc Author: None (llvmbot) ChangesBackport 15e421d Requested by: @nikic 1 Files Affected:
diff --git a/llvm/lib/MC/MCObjectStreamer.cpp b/llvm/lib/MC/MCObjectStreamer.cpp
index 94468140a30b9..5fd30eccb45c5 100644
--- a/llvm/lib/MC/MCObjectStreamer.cpp
+++ b/llvm/lib/MC/MCObjectStreamer.cpp
@@ -109,7 +109,9 @@ void MCObjectStreamer::addSpecialFragment(MCFragment *Frag) {
void MCObjectStreamer::appendContents(ArrayRef<char> Contents) {
ensureHeadroom(Contents.size());
assert(FragSpace >= Contents.size());
- llvm::copy(Contents, getCurFragEnd());
+ // As this is performance-sensitive code, explicitly use std::memcpy.
+ // Optimization of std::copy to memmove is unreliable.
+ std::memcpy(getCurFragEnd(), Contents.begin(), Contents.size());
CurFrag->FixedSize += Contents.size();
FragSpace -= Contents.size();
}
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MaskRay
approved these changes
Jan 22, 2026
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moved this from Needs Review
to Needs Merge
in LLVM Release Status
We've observed a compile-time regression in LLVM 22 when including large blobs. The root cause was that emitBytes() was copying bytes one-by-one, which is much slower than using memcpy for large objects. Optimization of std::copy to memmove is apparently much less reliable than one might think. In particular, when using a non-bleeding-edge libstdc++ (anything older than version 15), this does not happen if the types of the input and output iterators do not match (like here, where there is a signed/unsigned mismatch). As this code is performance sensitive, I think it makes sense to directly use memcpy. Previously this code used SmallVector::append, which explicitly uses memcpy. (cherry picked from commit 15e421d)
github-project-automation
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moved this from Needs Merge
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Backport 15e421d
Requested by: @nikic