Created attachment 53316 [details] Does not vectorize The following test case: void foo (uint32_t dst[8], uint8_t src1[8], uint8_t src2[8]) { uint16_t diff_e0 = src1[0] - src2[0]; uint16_t diff_e1 = src1[1] - src2[1]; uint16_t diff_e2 = src1[2] - src2[2]; uint16_t diff_e3 = src1[3] - src2[3]; uint16_t diff_e4 = src1[4] - src2[4]; uint16_t diff_e5 = src1[5] - src2[5]; uint16_t diff_e6 = src1[6] - src2[6]; uint16_t diff_e7 = src1[7] - src2[7]; uint32_t a0 = diff_e0 + 1; uint32_t a1 = diff_e1 + 3; uint32_t a2 = diff_e2 + 4; uint32_t a3 = diff_e3 + 2; uint32_t a4 = diff_e4 + 12; uint32_t a5 = diff_e5 + 11; uint32_t a6 = diff_e6 + 9; uint32_t a7 = diff_e7 + 3; dst[0] = a0; dst[1] = a1; dst[2] = a2; dst[3] = a3; dst[4] = a4; dst[5] = a5; dst[6] = a6; dst[7] = a7; } Produces nice vectorized code on aarch64: ldr d2, [x2] adrp x3, .LC0 ldr d0, [x1] ldr q1, [x3, #:lo12:.LC0] usubl v0.8h, v0.8b, v2.8b uaddl v2.4s, v0.4h, v1.4h uaddl2 v0.4s, v0.8h, v1.8h stp q2, q0, [x0] ret But if any of the constants is replaced with zero instead then scalar code is produced: ldrb w4, [x2, 1] ldrb w8, [x1, 1] ldrb w3, [x2, 3] ldrb w7, [x1, 3] sub w8, w8, w4 ldrb w6, [x1, 4] and w8, w8, 65535 ldrb w4, [x2, 4] sub w7, w7, w3 ldrb w5, [x1, 5] and w7, w7, 65535 ldrb w3, [x2, 5] sub w6, w6, w4 ldrb w9, [x2, 6] and w6, w6, 65535 ldrb w4, [x1, 6] sub w5, w5, w3 ldrb w10, [x2, 7] and w5, w5, 65535 ldrb w3, [x1, 7] sub w4, w4, w9 ldrb w11, [x2] and w4, w4, 65535 ldrb w9, [x1] sub w3, w3, w10 ldrb w2, [x2, 2] add w8, w8, 3 ldrb w10, [x1, 2] sub w9, w9, w11 and w1, w3, 65535 and w9, w9, 65535 sub w10, w10, w2 add w3, w5, 11 add w2, w4, 9 add w7, w7, 2 add w6, w6, 12 add w1, w1, 3 add w4, w9, 1 and w5, w10, 65535 stp w4, w8, [x0] stp w5, w7, [x0, 8] stp w6, w3, [x0, 16] stp w2, w1, [x0, 24] ret It would be possible to produce the same vectorized code as above but with zero in the constants. If I understand correctly, the identity element of addition is not taken into consideration in the SLP vectorizer, which could be improved. The same happens with subtraction. I can reproduce this in any recent version of GCC (e.g. >= 10). Vectorized case: https://godbolt.org/z/5sbb1an89 Scalar case: https://godbolt.org/z/v8jPT9jEe

Confirmed, it's quite odd. x86_64 is also affected: https://godbolt.org/z/q46z3hh9Y

Also an issue with multiply: void foo (unsigned *__restrict dst, unsigned *__restrict src1) { dst[0] = src1[0] * 1; dst[1] = src1[1] * 2; dst[2] = src1[2] * 3; dst[3] = src1[3] * 4; dst[4] = src1[4] * 5; dst[5] = src1[5] * 6; dst[6] = src1[6] * 7; dst[7] = src1[7] * 8; }

I should note that I noticed LLVM does not handle this either. Basically the following operators and values can be used: For integer: + 0 - 0 * 1 / 1 | 0 & -1 (all ones) ^ 0 For floating point (only with -ffast-math, I think sub can be used with 0 and add with -0.0 without but I am not 100% sure): + 0 - 0 * 1 / 1

(In reply to Andrew Pinski from comment #3) > I should note that I noticed LLVM does not handle this either. > > Basically the following operators and values can be used: > For integer: > + 0 > - 0 > * 1 > / 1 > | 0 > & -1 (all ones) > ^ 0 > > For floating point (only with -ffast-math, I think sub can be used with 0 > and add with -0.0 without but I am not 100% sure): > + 0 > - 0 > * 1 > / 1 Note for the following operators can support some constants which were there instead of a calculation (note this might be harder and maybe a different bug): op cst rhs * 0 0 | -1 -1 & 0 0

(In reply to Andrew Pinski from comment #3) > I should note that I noticed LLVM does not handle this either. > > Basically the following operators and values can be used: > For integer: > + 0 > - 0 > * 1 > / 1 > | 0 > & -1 (all ones) > ^ 0 > > For floating point (only with -ffast-math, I think sub can be used with 0 > and add with -0.0 without but I am not 100% sure): > + 0 > - 0 > * 1 > / 1 I think it should be possible to also consider the bit-shifts operations: >> 0 << 0

(In reply to Erick Ochoa from comment #5) > I think it should be possible to also consider the bit-shifts operations: > > >> 0 > << 0 Yes and rotates too.

SLP discovery doesn't support this (and there's for sure some duplicate bug about this). Note that SLP discovery currently does a greedy search from the stores and it commits to the first "working" graph (where "working" differs from loop vs. non-loop operation), opening up more "fixup" possibilities will also open up the chance for it to de-rail more easily.

*** Bug 112579 has been marked as a duplicate of this bug. ***