[PATCH] D59744: Fix i386 ABI "__m64" type bug

[Eli Friedman via Phabricator via llvm-commits]

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

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  https://reviews.llvm.org/D59744/new/

https://reviews.llvm.org/D59744