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[PATCH 0/3][AArch64] DImode vector compares


Comparing 64x1 vector types (defined by hand or from arm_neon.h) using GCC vector extensions currently generates very poor assembly code, for example "uint64x1_t foo (uint64x1_t a, uint64x1_t b) { return a >= b; }" generates (at -O3):

fmov x0, d0 // 22 movdi_aarch64/12 [length = 4]
fmov x1, d1 // 23 movdi_aarch64/12 [length = 4]
cmp x0, x1 // 10 cmpdi/1 [length = 4]
csinv x0, xzr, xzr, cc // 17 cmovdi_insn/3 [length = 4]
fmov d0, x0 // 24 *movdi_aarch64/11 [length = 4]
ret // 27 simple_return [length = 4]

Meaning that arm_neon.h instead has to use rather awkward forms like "return (uint64x1_t) {__a[0] >= __b[0] ? -1ll : 0ll};" to produce the desired assembly

cmhs d0, d0, d1

This series adds vcond(u?)didi patterns for AArch64, to generate appropriate RTL from direct comparisons of 64x1 vectors (which are of DImode). However, as things stand, adding a vconddidi pattern causes an ICE in vector_compare_rtx (maybe_legitimize_operands), because a DImode constant-zero (vector or otherwise) is expanded as const0_rtx, which has mode VOIDmode. I tried quite a bit to generate an RTL const_vector, or even just something with mode DImode, but without success, hence the first patch fixes vector_compare_rtx to use the mode from the tree if necessary. (DImode vectors are specifically allowed by stor-layout.c, but no other platform defines vconddidi.)

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