MCExpr::evaluateAsRelocatableImpl : allow evaluation of non-VK_None…
Audit Required96d40df71ece

Authored by MaskRay on Wed, Nov 18, 1:52 PM.


MCExpr::evaluateAsRelocatableImpl : allow evaluation of non-VK_None MCSymbolRefExpr when MCAsmLayout is available

(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