https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103771
--- Comment #1 from Tamar Christina <tnfchris at gcc dot gnu.org> --- Looks like the change causes the simpler conditional to be detected by the vectorizer as a masked operation, which in principle makes sense: note: vect_recog_mask_conversion_pattern: detected: iftmp.0_21 = x.1_14 > 255 ? iftmp.0_19 : iftmp.0_20; note: mask_conversion pattern recognized: patt_43 = patt_42 ? iftmp.0_19 : iftmp.0_20; note: extra pattern stmt: patt_40 = x.1_14 > 255; note: extra pattern stmt: patt_42 = (<signed-boolean:8>) patt_40; However not quite sure how the masking works on x86. The additional statement generated for patt_42 causes it to fail during vectorization: note: ==> examining pattern def statement: patt_42 = (<signed-boolean:8>) patt_40; note: ==> examining statement: patt_42 = (<signed-boolean:8>) patt_40; note: vect_is_simple_use: operand x.1_14 > 255, type of def: internal note: vect_is_simple_use: vectype vector(8) <signed-boolean:1> missed: conversion not supported by target. note: vect_is_simple_use: operand x.1_14 > 255, type of def: internal note: vect_is_simple_use: vectype vector(8) <signed-boolean:1> note: vect_is_simple_use: operand x.1_14 > 255, type of def: internal note: vect_is_simple_use: vectype vector(8) <signed-boolean:1> missed: not vectorized: relevant stmt not supported: patt_42 = (<signed-boolean:8>) patt_40; missed: bad operation or unsupported loop bound. note: ***** Analysis failed with vector mode V32QI as there's no conversion patterns for `VEC_UNPACK_LO_EXPR` between bool and a mask. which explains why it works for AVX2 and AVX512BW. AVX512F doesn't seem to allow any QI mode conversions [1] so it fails.. Not sure why it's doing the replacement without checking to see that the target is able to vectorize the statements it generates later. Specifically it doesn't check if what's returned by build_mask_conversion is supported or not. My guess is because vectorizable_condition will fail anyway without the type of the conditional being a vector boolean. With -mavx512vl V32QI seems to generate in the pattern mask conversions between vector (8) <signed-boolean:1> and without it vector(32) <signed-boolean:8>. I think some x86 person needs to give a hint here :) [1] https://www.felixcloutier.com/x86/kunpckbw:kunpckwd:kunpckdq
