[llvm-bugs] [Bug 34454] New: [X86] Failure to prevent schedule across x87 control word instructions

[via llvm-bugs]

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: unassignedbugs at nondot.org
          Reporter: llvm-dev at redking.me.uk
                CC: craig.topper at gmail.com, emaste at freebsd.org,
                    gadi.haber at intel.com, llvm-bugs at lists.llvm.org,
                    spatel+llvm at 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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