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Return early from getUnconditionalBranchTargetOpValue if the branch target is 

an MCExpr, in order to avoid writing an encoded zero value in the immediate
field.

When getUnconditionalBranchTargetOpValue is called with an MCExpr target, we
don't know what the final immediate field value should be. We shouldn't
explicitly set the immediate field to an encoded zero value as zero is encoded
with a non-zero bit pattern. This leads to bits being set that pollute the
final immediate value. The nature of the encoding is such that the polluted
bits only affect very large immediate values, explaining why this hasn't
caused problems earlier.

Fixes <rdar://problem/15155975>.

llvm-svn: 193535
This commit is contained in:
Lang Hames 2013-10-28 20:51:11 +00:00
parent 889a180e5a
commit b52816615b
2 changed files with 3 additions and 3 deletions
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2
llvm/lib/Target/ARM/MCTargetDesc/ARMMCCodeEmitter.cpp
View File

@ -641,7 +641,7 @@ getUnconditionalBranchTargetOpValue(const MCInst &MI, unsigned OpIdx,
const MCOperand MO = MI.getOperand(OpIdx);
if(MO.isExpr())
Val = ::getBranchTargetOpValue(MI, OpIdx, ARM::fixup_t2_uncondbranch, Fixups);
return ::getBranchTargetOpValue(MI, OpIdx, ARM::fixup_t2_uncondbranch, Fixups);
else
Val = MO.getImm() >> 1;

4
llvm/test/MC/ARM/basic-thumb2-instructions.s
View File

@ -226,12 +226,12 @@ _func:
beq.w _bar
bmi.w #-183396
@ CHECK: b.w _bar @ encoding: [A,0xf0'A',A,0xb8'A']
@ CHECK: b.w _bar @ encoding: [A,0xf0'A',A,0x90'A']
@ fixup A - offset: 0, value: _bar, kind: fixup_t2_uncondbranch
@ CHECK: beq.w _bar @ encoding: [A,0xf0'A',A,0x80'A']
@ fixup A - offset: 0, value: _bar, kind: fixup_t2_condbranch
@ CHECK: it eq @ encoding: [0x08,0xbf]
@ CHECK: beq.w _bar @ encoding: [A,0xf0'A',A,0xb8'A']
@ CHECK: beq.w _bar @ encoding: [A,0xf0'A',A,0x90'A']
@ fixup A - offset: 0, value: _bar, kind: fixup_t2_uncondbranch
@ CHECK: bmi.w #-183396 @ encoding: [0x13,0xf5,0xce,0xa9]