[llvm-bugs] [Bug 42319] New: [X86] Add pass to insert EMMS/FEMMS instructions to separate MMX and X87 states

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Jun 19 03:25:19 PDT 2019


            Bug ID: 42319
           Summary: [X86] Add pass to insert EMMS/FEMMS instructions to
                    separate MMX and X87 states
           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, efriedma at quicinc.com,
                    hans at chromium.org, llvm-bugs at lists.llvm.org,
                    llvm-dev at redking.me.uk, nicolasweber at gmx.de,
                    rjmccall at apple.com, rnk at google.com,
                    spatel+llvm at rotateright.com, wei3.xiao at intel.com
        Depends on: 35982
            Blocks: 41029

As discussed on https://reviews.llvm.org/D59744, we currently have no way to
automatically separate MMX and x87 instructions - we rely on manual insertion
of _mm_empty() (EMMS) intrinsics.

Ideally we need something like the X86VZeroUpper pass which can recognise when
MMX/X87 instructions have been used, insert EMMS/FEMMS instructions where
appropriate and ensure that MMX/X87 ops don't cross the barrier (see Bug

Given the high cost of EMMS, we may want to make this pass opt-in - for example
only enable it by default for the i386 ABI change (see Bug #41029).

Referenced Bugs:

[Bug 35982] [MMX] POSTRAScheduler disarrange emms and mmx instruction
[Bug 41029] The __m64 not passed according to i386 ABI
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