[all-commits] [llvm/llvm-project] 96d40d: MCExpr::evaluateAsRelocatableImpl : allow evaluati...

[Fangrui Song via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 96d40df71ecee07c69aea512f6c04fc4fbe6acfb
      https://github.com/llvm/llvm-project/commit/96d40df71ecee07c69aea512f6c04fc4fbe6acfb
  Author: Fangrui Song <i at maskray.me>
  Date:   2020-11-18 (Wed, 18 Nov 2020)

  Changed paths:
    M llvm/lib/MC/MCExpr.cpp
    M llvm/test/MC/ARM/ehabi-personality-abs.s
    A llvm/test/MC/ELF/relocation-alias.s
    M llvm/test/MC/ELF/relocation.s

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

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 at 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 at 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 at PLT` which seems nonsensical, so we don't allow it.

`MC/PowerPC/pr38945.s` `NUMBER = 0x6ffffff9; cmpwi 8,NUMBER at l` requires the
`symbol at 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