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[IRGen] Don't use GOTPCREL relocations for x86 ELF. #82176

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Jun 13, 2025
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[IRGen] Don't use GOTPCREL relocations for x86 ELF. #82176

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6 changes: 4 additions & 2 deletions lib/IRGen/GenDecl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -3736,13 +3736,15 @@ static llvm::GlobalVariable *createGOTEquivalent(IRGenModule &IGM,
llvm::GlobalValue::PrivateLinkage,
global,
llvm::Twine("got.") + globalName);

// rdar://problem/53836960: i386 ld64 also mis-links relative references
// to GOT entries.
// rdar://problem/59782487: issue with on-device JITd expressions.
// The JIT gets confused by private vars accessed across object files.
// rdar://148168098: ELF x86 GOTPCREL relaxation can break metadata.
if (!IGM.getOptions().UseJIT &&
(!IGM.Triple.isOSDarwin() || IGM.Triple.getArch() != llvm::Triple::x86)) {
(!IGM.Triple.isOSDarwin() || IGM.Triple.getArch() != llvm::Triple::x86) &&
(!IGM.Triple.isOSBinFormatELF() || !IGM.Triple.isX86())) {
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Do you happen to know offhand if the linker on other ELF platforms (like FreeBSD) is also affected by this bug?

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@al45tair al45tair Jun 11, 2025 •
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FreeBSD uses lld, which does have this problem (and, unlike the BFD linker, while it understands -no-relax and won't complain about it, it ignores it for the purposes of this particular optimisation).

Basically, lld, gold and the BFD linker all do this; only the BFD linker has a way to disable it, and none of them test whether they're looking at a code section first. They do all have a handful of things they exempt from this behaviour (though the exact set differs; BFD and lld have more exceptions than gold does), but sadly none of them check whether they're in a code section first.

I'm aware that in principle there might be a platform using some other ELF linker that doesn't do this, but even in that case, someone might choose to use one of the three linkers above that all do.

(It looks to me like mold may be such a linker, just at a quick glance.)

gotEquivalent->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global);
} else {
ApplyIRLinkage(IRLinkage::InternalLinkOnceODR)
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