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https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;h=4acf8c78e659833be8be047ba2f8561386a11d4b (1994) introduced this behavior: if a fixup symbol is equated to an expression with an undefined symbol, convert the fixup to be against the target symbol. glibc relies on this behavior to perform assembly level indirection ``` asm("memcpy = __GI_memcpy"); // from sysdeps/generic/symbol-hacks.h ... // call memcpy@PLT // The relocation references __GI_memcpy in GNU as, but memcpy in MC (without the patch) memcpy (...); ``` (1) It complements `extern __typeof(memcpy) memcpy asm("__GI_memcpy");` The frontend asm label does not redirect synthesized memcpy in the middle-end. (See D88712 for details) (2) `asm("memcpy = __GI_memcpy");` is in every translation unit, but the memcpy declaration may not be visible in the translation unit where memcpy is synthesized. MC already redirects `memcpy = __GI_memcpy; call memcpy` but not `memcpy = __GI_memcpy; call memcpy@plt`. This patch fixes the latter by allowing MCExpr::evaluateAsRelocatableImpl to evaluate a non-VK_None MCSymbolRefExpr, which is only done after the layout is available. GNU as allows `memcpy = __GI_memcpy+1; call memcpy@PLT` which seems nonsensical, so we don't allow it. `MC/PowerPC/pr38945.s` `NUMBER = 0x6ffffff9; cmpwi 8,NUMBER@l` requires the `symbol@l` form in AsmMatcher, so evaluation needs to be deferred. This is the place whether future simplification may be possible. Note, if we suppress the VM_None evaluation when MCAsmLayout is nullptr, we may lose the `invalid reassignment of non-absolute variable` diagnostic (`ARM/thumb_set-diagnostics.s` and `MC/AsmParser/variables-invalid.s`). We know that this diagnostic is troublesome in some cases (https://github.com/ClangBuiltLinux/linux/issues/1008), so we can consider making simplification in the future. Reviewed By: jyknight Differential Revision: https://reviews.llvm.org/D88625
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