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

Tue Jun 18 08:27:03 PDT 2019

mtklein added a comment.

Hey folks, I'm the Skia point of contact on this, and "luckily" the person who wrote all the code that got us into this mess.  Let me cross post a couple questions I've had from the Chromium bug over here where folks might know the answer...

Now that Clang's decided to match GCC's behavior of using mm0 to pass around 8-byte vectors on x86-32, is there any way to use 8-byte vector types safely any more?  I don't really have the full context of this Clang change, but is it maybe a good idea applied to too many types?  I notice the change mentions __m64, but here I'm using uint16_t ext_vector_type(4) exclusively, never __m64 or even an 8x8 vector... can we just squint and say u16x4 and __m64 aren't the same, passing __m64 according to the ABI but vector extensions however we were doing it before?  Or can we work out some sort of ABI that preserves st0/mm0?  I think we're finding that even with forced-inlining, at -O0 we still end up getting u16x4 values stored in mm0 briefly (kind of roundabout through xmm registers and the stack once or twice too).

In short, should working with 4x u16 be safe on x86-32 and there's a bug / undefined behavior in my code leading to this  mm0/st0 clobber, or is this just actually not really spec'd to work?

Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D59744/new/

https://reviews.llvm.org/D59744