On 4/16/14 1:11 AM, Simon Sapin wrote:
On 15/04/2014 06:00, Patrick Walton wrote:
Hi everyone,
I'm considering using 32-bit fixed-point (16 bits for the fraction, 16
bits for the integer portion) for percentages in CSS. The reason is that
we can use the following four SSE4/AVX instructions on x86 to compute
two sides at once:
; xmm0 contains the style values (percentage or fixed length)
; xmm1 contains ~0 if the value is a percentage or 0 if the value
; is fixed
; xmm2 contains the value the percentages are relative to
0x0000000100000b68 <+8>: vpshufd xmm2,xmm2,0x0
0x0000000100000b6d <+13>: vpmulld xmm2,xmm2,xmm0
0x0000000100000b72 <+18>: vpsrad xmm2,xmm2,0x10
0x0000000100000b77 <+23>: vblendvps xmm0,xmm0,xmm2,xmm1
; result in xmm0
Is the plan to somehow convince LLVM to emit these instructions, or to
have Assembly source code in Servo? If the latter, does this mean a
second code path for other architectures? Should we have a way to test
both, like we do for CPU vs. GPU rendering?
I think we should use the Rust SIMD proposal to get cross-platform
support [1]. This leverages the built-in LLVM support for SIMD
instructions. LLVM will happily emit loops if the vectorized
instructions are not available.
Patrick
[1]: https://github.com/rust-lang/rfcs/pull/15
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