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PPC: Add a better comment about the i64 FI fixup

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In discussing this change with Bill Schmidt, it was decided that the original
comment about negative FIs was incorrect. We'll still exclude them for now, but
now with a more-accurate explanation.

llvm-svn: 186005
This commit is contained in:
Hal Finkel 2013-07-10 15:29:01 +00:00
parent cbe40f5bcd
commit b6802c0a3e
1 changed files with 13 additions and 2 deletions
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15
lib/Target/PowerPC/PPCISelLowering.cpp
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@ -1043,8 +1043,19 @@ static void fixupFuncForFI(SelectionDAG &DAG, int FrameIdx, EVT VT) {
if (VT != MVT::i64)
return;
// This should not be needed for negative FIs, which come from argument
// lowering, because the ABI should guarentee the necessary alignment.
// NOTE: We'll exclude negative FIs here, which come from argument
// lowering, because there are no known test cases triggering this problem
// using packed structures (or similar). We can remove this exclusion if
// we find such a test case. The reason why this is so test-case driven is
// because this entire 'fixup' is only to prevent crashes (from the
// register scavenger) on not-really-valid inputs. For example, if we have:
// %a = alloca i1
// %b = bitcast i1* %a to i64*
// store i64* a, i64 b
// then the store should really be marked as 'align 1', but is not. If it
// were marked as 'align 1' then the indexed form would have been
// instruction-selected initially, and the problem this 'fixup' is preventing
// won't happen regardless.
if (FrameIdx < 0)
return;