From 199bea0fd80e65178a9d12c705a9f0aaf0a36ceb Mon Sep 17 00:00:00 2001 From: Boris Brezillon Date: Wed, 12 Aug 2020 14:28:48 +0200 Subject: [PATCH] nir: Fix i64tof32 lowering The round-to-nearest-even implementation found in lower_2f() is incorrect for any value having a significand that is not directly representable and whose non-representable part lies between 1 and half the minimum representable value. In this case, the significand is rounded up instead of being rounded down. Fixes: 936c58c8fcce ("nir: Extend nir_lower_int64() to support i2f/f2i lowering") Signed-off-by: Boris Brezillon Tested-by: Jesse Natalie Acked-by: Matt Turner Part-of: --- src/compiler/nir/nir_lower_int64.c | 47 +++++++++++++----------------- 1 file changed, 21 insertions(+), 26 deletions(-) diff --git a/src/compiler/nir/nir_lower_int64.c b/src/compiler/nir/nir_lower_int64.c index 03d7c156807..9d268a84363 100644 --- a/src/compiler/nir/nir_lower_int64.c +++ b/src/compiler/nir/nir_lower_int64.c @@ -714,35 +714,30 @@ lower_2f(nir_builder *b, nir_ssa_def *x, unsigned dest_bit_size, unreachable("Invalid dest_bit_size"); } - /* We keep one more bit than can fit in the significand field to let the - * u2f32 conversion do the rounding for us. - */ nir_ssa_def *discard = - nir_imax(b, nir_isub(b, exp, nir_imm_int(b, significand_bits + 1)), + nir_imax(b, nir_isub(b, exp, nir_imm_int(b, significand_bits)), nir_imm_int(b, 0)); - - /* Part of the "round to nearest" has to be taken care of before we discard - * the LSB, and that's what this extra iadd is for. - * "Round to nearest even" is handled by u2f. That works because the - * shifted value either fits in the significand field (which means no - * rounding is required) or contains one extra bit that forces the - * conversion op to round things properly. - */ - nir_ssa_def *add = COND_LOWER_OP(b, ishl, nir_imm_int64(b, 1), discard); - add = COND_LOWER_OP(b, isub, add, nir_imm_int64(b, 1)); - nir_ssa_def *rounded_x = COND_LOWER_OP(b, iadd, x, add); - - /* Signed Values can't overflow because we've saved the sign and promoted - * them to unsigned values. + nir_ssa_def *significand = + COND_LOWER_CAST(b, u2u32, COND_LOWER_OP(b, ushr, x, discard)); + + /* Round-to-nearest-even implementation: + * - if the non-representable part of the significand is higher than half + * the minimum representable significand, we round-up + * - if the non-representable part of the significand is equal to half the + * minimum representable significand and the representable part of the + * significand is odd, we round-up + * - in any other case, we round-down */ - if (!src_is_signed) { - nir_ssa_def *overflow = COND_LOWER_CMP(b, ult, rounded_x, x); - rounded_x = COND_LOWER_OP(b, bcsel, overflow, - nir_imm_int64(b, UINT64_MAX), rounded_x); - } - - nir_ssa_def *significand = COND_LOWER_OP(b, ushr, rounded_x, discard); - significand = COND_LOWER_CAST(b, u2u32, significand); + nir_ssa_def *lsb_mask = COND_LOWER_OP(b, ishl, nir_imm_int64(b, 1), discard); + nir_ssa_def *rem_mask = COND_LOWER_OP(b, isub, lsb_mask, nir_imm_int64(b, 1)); + nir_ssa_def *half = COND_LOWER_OP(b, ishr, lsb_mask, nir_imm_int(b, 1)); + nir_ssa_def *rem = COND_LOWER_OP(b, iand, x, rem_mask); + nir_ssa_def *halfway = nir_iand(b, COND_LOWER_CMP(b, ieq, rem, half), + nir_ine(b, discard, nir_imm_int(b, 0))); + nir_ssa_def *is_odd = nir_i2b(b, nir_iand(b, significand, nir_imm_int(b, 1))); + nir_ssa_def *round_up = nir_ior(b, COND_LOWER_CMP(b, ilt, half, rem), + nir_iand(b, halfway, is_odd)); + significand = nir_iadd(b, significand, nir_b2i32(b, round_up)); nir_ssa_def *res; -- 2.30.2