https://bugs.llvm.org/show_bug.cgi?id=34454
Bug ID: 34454
Summary: [X86] Failure to prevent schedule across x87 control
word instructions
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: llvm-...@redking.me.uk
CC: craig.top...@gmail.com, ema...@freebsd.org,
gadi.ha...@intel.com, llvm-bugs@lists.llvm.org,
spatel+l...@rotateright.com
Branched off from [Bug #34080].
The partial SandyBridge schedule model (which is used for all default x86-64
targets) resulted in the scheduler moving some instructions across fldcw/fnstcw
instructions, affecting rounding of truncated stores etc.
The issue appears to be that we're incorrectly handling the control word
register dependency, which is odd because it looks like fldcw/fnstcw are
correctly flagged to cause side effects.
It could also be a chain issue in the fptosi store code in
X86TargetLowering::EmitInstrWithCustomInserter but I didn't notice anything.
Exposing this is tricky, and seems to involve applying rL307529 but not
rL310792.
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