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Accept subtractions involving a weak symbol. 

When a symbol S shows up in an expression in assembly there are two
possible interpretations
* The expression is referring to the value of S in this file.
* The expression is referring to the value after symbol resolution.

In the first case the assembler can reason about the value and try to
produce a relocation.
In the second case, that is only possible if the symbol cannot be
preempted.

Assemblers are not very consistent about which interpretation gets used.
This changes MC to agree with GAS in the case of an expression of the
form "Sym - WeakSym".

llvm-svn: 258329
This commit is contained in:
Rafael Espindola 2016-01-20 18:57:48 +00:00
parent 18b1de3ec6
commit b718237dfc
4 changed files with 6 additions and 12 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
llvm/lib/MC/ELFObjectWriter.cpp
View File

@ -655,12 +655,6 @@ void ELFObjectWriter::recordRelocation(MCAssembler &Asm,
return;
}
if (::isWeak(SymB)) {
Ctx.reportError(Fixup.getLoc(),
"Cannot represent a subtraction with a weak symbol");
return;
}
uint64_t SymBOffset = Layout.getSymbolOffset(SymB);
uint64_t K = SymBOffset - FixupOffset;
IsPCRel = true;

3
llvm/test/MC/AArch64/error-location.s
View File

@ -16,9 +16,6 @@
// CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: Cannot represent a difference across sections
.word x_a - y_a
// CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: Cannot represent a subtraction with a weak symbol
.word a - w
// CHECK: <unknown>:0: error: expression could not be evaluated
.set v1, -undef

3
llvm/test/MC/ARM/error-location.s
View File

@ -16,9 +16,6 @@
@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: Cannot represent a difference across sections
.word x_a - y_a
@ CHECK: :[[@LINE+1]]:{{[0-9]+}}: error: Cannot represent a subtraction with a weak symbol
.word a - w
@ CHECK: <unknown>:0: error: expression could not be evaluated
.set v1, -undef

6
llvm/test/MC/ELF/relocation.s
View File

@ -63,6 +63,11 @@ pr24486:
.code16
call pr23771
.weak weak_sym
weak_sym:
.long pr23272-weak_sym
// CHECK: Section {
// CHECK: Name: .rela.text
// CHECK: Relocations [
@ -101,5 +106,6 @@ pr24486:
// CHECK-NEXT: 0xDC R_X86_64_PLT32 foo 0x0
// CHECK-NEXT: 0xF0 R_X86_64_32 .text 0xF0
// CHECK-NEXT: 0xF5 R_X86_64_PC16 pr23771 0xFFFFFFFFFFFFFFFE
// CHECK-NEXT: 0xF7 R_X86_64_PC32 pr23272 0x0
// CHECK-NEXT: ]
// CHECK-NEXT: }