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Fix i32x4.relaxed_dot_i8x16_i7x16_add_s - #2312
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The spec previously computed the first pairwise dot product only in 16-bit arithmetic (via i16x8.relaxed_dot), allowing just signed operands with a wrapping 16-bit sum or unsigned operands with a saturating 16-bit sum. This did not permit hardware that accumulates directly in 32 bits (ARM64 sdot, x86-64 vpdpbssd/vpdpbusd). Add a relaxed parameter R_idot_add that selects a 16-bit or 32-bit intermediate pairwise sum, independently of the operand signedness selected by R_idot. Also fix the reference interpreter, which used signed operands with a saturating 16-bit sum, a combination allowed by neither the old nor the new semantics. It is now defined in terms of i16x8.relaxed_dot and extadd_pairwise_s.
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The spec previously computed the first pairwise dot product only in 16-bit arithmetic (via i16x8.relaxed_dot), allowing just signed operands with a wrapping 16-bit sum or unsigned operands with a saturating 16-bit sum. This did not permit hardware that accumulates directly in 32 bits (ARM64 sdot, x86-64 vpdpbssd/vpdpbusd).
Add a relaxed parameter R_idot_add that selects a 16-bit or 32-bit intermediate pairwise sum, independently of the operand signedness selected by R_idot.
Also fix the reference interpreter, which used signed operands with a saturating 16-bit sum, a combination allowed by neither the old nor the new semantics. It is now defined in terms of i16x8.relaxed_dot and extadd_pairwise_s.