[llvm-bugs] [Bug 31284] New: Constrained FP operations should have implicit use/def for FP environment registers

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https://llvm.org/bugs/show_bug.cgi?id=31284

            Bug ID: 31284
           Summary: Constrained FP operations should have implicit use/def
                    for FP environment registers
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Core LLVM classes
          Assignee: unassignedbugs at nondot.org
          Reporter: andrew.kaylor at intel.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

This is a pre-emptive bug for the implementation of constrained FP intrinsics. 
When these intrinsics are translated to machine instructions, an implicit use
and/or def of target-specific FP environment registers should be added to the
instructions selected.  These registers aren't currently modeled for FP
instructions, and for the default, non-constrained case it probably makes sense
to continue not modeling them, but when the constrained intrinsics are used
these accesses need to be modeled to prevent unwanted code motion.

For example, if a program uses an intrinsic such as llvm.x86.sse.ldmxcsr to
change the rounding mode we need to be certain that FP operations are not moved
across the LDMXCSR instruction.

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