The variable swizzle on intel can use PSHUFB to shuffle the bytes but the mask vector must first be sanitized so that out-of-range lanes in the mask have the high bit set. Currently we use a compare-with-constant-and-POR to do this (and we don't even inline the constant load in the compare, sigh) but it's possible to do better by saturating-add'ing a constant into the mask: https://github.com/WebAssembly/simd/issues/68#issuecomment-470825324 For specific code generation, I'm not sure if it's better to (a) splat a byte value into scratch / load the constant into scratch, and add the mask to the scratch, or (b) to move the mask to scratch and add a constant from memory into scratch. Either way the mask register is not volatile.
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The variable swizzle on intel can use PSHUFB to shuffle the bytes but the mask vector must first be sanitized so that out-of-range lanes in the mask have the high bit set. Currently we use a compare-with-constant-and-POR to do this (and we don't even inline the constant load in the compare, sigh) but it's possible to do better by saturating-add'ing a constant into the mask: https://github.com/WebAssembly/simd/issues/68#issuecomment-470825324 For specific code generation, I'm not sure if it's better to (a) splat a byte value into scratch / load the constant into scratch, and add the mask to the scratch, or (b) to move the mask to scratch and add a constant from memory into scratch. Either way the mask register is not volatile. Also see https://github.com/WebAssembly/simd/issues/93 for more discussion.
The variable swizzle on intel can use PSHUFB to shuffle the bytes but the mask vector must first be sanitized so that out-of-range lanes in the mask have the high bit set. Currently we use a compare-with-constant-and-POR to do this (and we don't even inline the constant load in the compare, sigh) but it's possible to do better by saturating-add'ing a constant into the mask: https://github.com/WebAssembly/simd/issues/68#issuecomment-470825324 For specific code generation, I'm not sure if it's better to (a) splat a byte value into scratch / load the constant into scratch, and add the mask to the scratch, or (b) to move the mask to scratch and add a constant from memory into scratch. Either way the mask register is not volatile. Also see https://github.com/WebAssembly/simd/issues/93 for more discussion, probably worth reading although it ranges across a bunch of topics.