relaxed i32x4.trunc_sat_f32x4_{s,u} i32x4.trunc_sat_f64x2_{s,u}_zero

[ngzhian]

1. What are the instructions being proposed?

Relaxed versions of:

• i32x4.trunc_sat_f32x4_s
• i32x4.trunc_sat_f32x4_u
• i32x4.trunc_sat_f64x2_s_zero
• i32x4.trunc_sat_f64x2_u_zero

from Simd128. (Names undecided)

2. What are the semantics of these instructions?

Convert f32x4/f64x2 to i32x4 with truncation (signed/unsigned). If the inputs are out of range or NaNs, the result is implementation-defined.

3. How will these instructions be implemented? Give examples for at least
   x86-64 and ARM64. Also provide reference implementation in terms of 128-bit
   Wasm SIMD.

x86/64

relaxed i32x4.trunc_sat_f32x4_s = CVTTPS2DQ
relaxed i32x4.trunc_sat_f32x4_u = VCVTTPS2UDQ (AVX512), Simd128 i32x4.trunc_sat_f32x4_u otherwise (can be slightly optimized to ignore NaNs)
relaxed i32x4.trunc_sat_f64x2_s_zero = CVTTPD2DQ
relaxed i32x4.trunc_sat_f64x2_u_zero = VCVTTPD2UDQ (AVX512), Simd128 i32x4.trunc_sat_f64x2_u_zero

ARM64

relaxed i32x4.trunc_sat_f32x4_s = FCVTZS
relaxed i32x4.trunc_sat_f32x4_u = FCVTZU
relaxed i32x4.trunc_sat_f64x2_s_zero = FCVTZS + SQXTN
relaxed i32x4.trunc_sat_f64x2_u_zero = FCVTZU + UQXTN

ARM NEON

relaxed i32x4.trunc_sat_f32x4_s = vcvt.S32.F32
relaxed i32x4.trunc_sat_f32x4_u = vcvt.U32.F32
relaxed i32x4.trunc_sat_f64x2_s_zero = vcvt.S32.F64 + vcvt.S32.F64 + vmov
relaxed i32x4.trunc_sat_f64x2_u_zero = vcvt.U32.F64 + vcvt.U32.F64 + vmov

Note: On ARM MVE, double precision conversions require Armv8-M Floating-point Extension (FPv5), MVE can be implemented with or without such an extension.

simd128

respective non-relaxed versions i32x4.trunc_sat_f32x4_s, i32x4.trunc_sat_f32x4_u, i32x4.trunc_sat_f64x2_s_zero, i32x4.trunc_sat_f64x2_u_zero.

4. How does behavior differ across processors? What new fingerprinting surfaces will be exposed?

For i32x4.trunc_sat_f32x4_s:

• x86/64 will return 0x8000000 in lanes for out of range or NaNs
• ARM/ARM64 will return 0 for NaNs and saturated results of out of range

For i32x4.trunc_sat_f32x4_u:

• x86/64 will return 0xFFFFFFFF in lanes for out of range or NaNs, if if AVX512 is available, 0 otherwise (but require more instruction counts)
• ARM/ARM64 will return 0 for NaNs and saturated results of out of range

For i32x4.trunc_sat_f64x2_s_zero:

• x86/64, 0x80000000 for out of range or NaNs
• ARM/ARM64 will return 0 for NaNs and saturated results of out of range

For i32x4.trunc_sat_f64x2_u_zero:

• x86/64, 0xFFFFFFFF for out of range or NaNs if AVX512 is available, 0 otherwise
• ARM/ARM64 will return 0 for NaNs and saturated results of out of range

5. What use cases are there?

Conversion instructions are common, if the application can guarantee the input range we can get good performance on all architectures.

[Maratyszcza]

IIRC @zeux had a use-case for these instructions.

It would be useful to consider f64x2 variants in the same proposal.

[Maratyszcza]

For i32x4.trunc_sat_f32x4_u, it will depend on implementation choice on x86/64:

• if AVX512 is available, same as above, x86/64 will return 0x8000000 in lanes for out of range or NaNs, ARM/ARM64 will return 0

AVX512 version returns 0xFFFFFFFF

[ngzhian]

AVX512 version returns 0xFFFFFFFF

Corrected, thanks!

[ngzhian]

It would be useful to consider f64x2 variants in the same proposal.

relaxed i64x2.trunc_sat_f64x2_{s,u}? We don't have these instructions in Simd128, so I think it is neater to separate them out.

[Maratyszcza]

relaxed i64x2.trunc_sat_f64x2_{s,u}? We don't have these instructions in Simd128, so I think it is neater to separate them out.

The WebAssembly/simd#383 instructions

[ngzhian]

i32x4.trunc_sat_f64x2_u_zero and i32x4.trunc_sat_f64x2_s_zero?

[Maratyszcza]

Yes

[zeux]

Yeah this one is pretty fundamental for many workflows, e.g. in rendering domains it's common to store data as fixed-point integers for GPU consumption but to prepare this data you do some math in floating point and then convert to integer via smth like int(v * 65535.0f + 0.5f) (assuming the value is known to be positive); the float->int truncation can be pretty hot based on the amount of other computation.

It would be nice to also include the rounding variants (on x64 assuming default rounding mode setup you can use cvtps2dq for rounding conversion and cvttps2dq for truncating; unsure what floating point environment is typically used in browser context, if it's undefined then rounding would require vroundps before cvttps).

[yurydelendik]

What will be the exact recipe for relaxed i32x4.trunc_sat_f32x4_u for x86/64 without AVX512? The comment at #247 suggests somewhat long version.

Is the following acceptable or the shorter version exists?

• y = relaxed i32x4.trunc_sat_f32x4_u(x) is lowered to:
  • MOVAPD xmm_y, xmm_x
  • MOVAPD xmm_tmp, [wasm_i32x4_splat(0x4f000000)]
  • CMPLTPS xmm_tmp, xmm_x
  • PAND xmm_tmp, xmm_x
  • PXOR xmm_y, xmm_tmp
  • CVTTPS2DQ xmm_y, xmm_y
  • PSLLD xmm_tmp, 7
  • PADDD xmm_y, xmm_tmp

[ngzhian]

it will be CVTTPS2DQ. The relaxed version only guarantees output when inputs are < INT32_MAX and not NaN, which is exactly what CVTTPS2DQ is, which is available since SSE2.