efriedma added a comment. > Are you saying that using MMX in LLVM requires source-level workarounds in > some way, and so we can't lower portable code to use MMX because that code > will (reasonably) lack those workarounds?
Yes. The x86 architecture requires that a program executes an "emms" instruction between any MMX instructions, and any x87 instructions. Otherwise, the x87 instructions will produce nonsense results. LLVM, and other compilers, never insert emms automatically; this is partially historical, but also because emms can be expensive on Intel chips. Instead, the user is expected to call _mm_empty() in appropriate places. To allow users to generate arbitrary vector IR without tripping over this, LLVM does not lower vector IR to MMX instructions; instead, it only generates MMX instructions for operations using the special type x86_mmx. If any instruction or argument has a result or operand of type x86_mmx in LLVM IR, the user must explicitly execute emms (@llvm.x86.mmx.emms() in IR, _mm_empty() in C) between that instruction, and any code that might use x87 registers. "Between" isn't really sound because emms intrinsic doesn't reliably prevent code motion of floating-point operations, but it works well enough in practice. (See also https://bugs.llvm.org/show_bug.cgi?id=35982 .) On the clang side, without this patch, we only generate code using the x86_mmx type in a couple places: _mm_* calls, and inline asm with an MMX operand. If the user does not use either of those, there will never be any values of type x86_mmx, so there will never be any MMX instructions, and we avoid the whole mess. 64-bit vector operations get lowered to SSE2 instructions instead (or scalarized). This patch introduces a new place where clang will generate the type x86_mmx: for call arguments and return values. This means more places where the user is required to write _mm_empty() to get correct behavior. Repository: rL LLVM CHANGES SINCE LAST ACTION https://reviews.llvm.org/D59744/new/ https://reviews.llvm.org/D59744 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
