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nir/lower_idiv: Port recent LLVM fixes to emit_udiv

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This change fixes off-by-one results in corner cases such as
0xffffffff / 0x11111111. For details refer to LLVM bug 46212.

Fixes: 8b98d0954e ('nir/lower_idiv: add new llvm-based path')
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6281>
(cherry picked from commit 8277334f39)
This commit is contained in:
Tony Wasserka 2020-08-11 16:25:37 +02:00 • committed by Dylan Baker
parent a19dd637e8
commit 9adbc862f7
2 changed files with 25 additions and 24 deletions
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2
.pick_status.json
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@ -1660,7 +1660,7 @@
"description": "nir/lower_idiv: Port recent LLVM fixes to emit_udiv",
"nominated": true,
"nomination_type": 1,
"resolution": 0,
"resolution": 1,
"master_sha": null,
"because_sha": "8b98d0954e6168484479cf51d56bface448d00d5"
},

47
src/compiler/nir/nir_lower_idiv.c
View file

@ -139,34 +139,35 @@ static nir_ssa_def *
emit_udiv(nir_builder *bld, nir_ssa_def *numer, nir_ssa_def *denom, bool modulo)
{
nir_ssa_def *rcp = nir_frcp(bld, nir_u2f32(bld, denom));
rcp = nir_f2u32(bld, nir_fmul_imm(bld, rcp, 4294967296.0));
nir_ssa_def *rcp_lo = nir_imul(bld, rcp, denom);
nir_ssa_def *rcp_hi = nir_umul_high(bld, rcp, denom);
nir_ssa_def *rcp_hi_ne_zero = nir_ine(bld, rcp_hi, nir_imm_int(bld, 0));
nir_ssa_def *neg_rcp_lo = nir_ineg(bld, rcp_lo);
nir_ssa_def *abs_rcp_lo = nir_bcsel(bld, rcp_hi_ne_zero, rcp_lo, neg_rcp_lo);
nir_ssa_def *e = nir_umul_high(bld, abs_rcp_lo, rcp);
nir_ssa_def *rcp_plus_e = nir_iadd(bld, rcp, e);
nir_ssa_def *rcp_minus_e = nir_isub(bld, rcp, e);
nir_ssa_def *tmp0 = nir_bcsel(bld, rcp_hi_ne_zero, rcp_minus_e, rcp_plus_e);
nir_ssa_def *quotient = nir_umul_high(bld, tmp0, numer);
rcp = nir_f2u32(bld, nir_fmul_imm(bld, rcp, 4294966784.0));
nir_ssa_def *neg_rcp_times_denom =
nir_imul(bld, rcp, nir_ineg(bld, denom));
rcp = nir_iadd(bld, rcp, nir_umul_high(bld, rcp, neg_rcp_times_denom));
/* Get initial estimate for quotient/remainder, then refine the estimate
* in two iterations after */
nir_ssa_def *quotient = nir_umul_high(bld, numer, rcp);
nir_ssa_def *num_s_remainder = nir_imul(bld, quotient, denom);
nir_ssa_def *remainder = nir_isub(bld, numer, num_s_remainder);
nir_ssa_def *remainder_ge_den = nir_uge(bld, remainder, denom);
nir_ssa_def *remainder_ge_zero = nir_uge(bld, numer, num_s_remainder);
nir_ssa_def *tmp1 = nir_iand(bld, remainder_ge_den, remainder_ge_zero);
/* First refinement step */
nir_ssa_def *remainder_ge_den = nir_uge(bld, remainder, denom);
if (!modulo) {
quotient = nir_bcsel(bld, remainder_ge_den,
nir_iadd_imm(bld, quotient, 1), quotient);
}
remainder = nir_bcsel(bld, remainder_ge_den,
nir_isub(bld, remainder, denom), remainder);
/* Second refinement step */
remainder_ge_den = nir_uge(bld, remainder, denom);
if (modulo) {
nir_ssa_def *rem = nir_bcsel(bld, tmp1,
nir_isub(bld, remainder, denom), remainder);
return nir_bcsel(bld, remainder_ge_zero,
rem, nir_iadd(bld, remainder, denom));
return nir_bcsel(bld, remainder_ge_den, nir_isub(bld, remainder, denom),
remainder);
} else {
nir_ssa_def *one = nir_imm_int(bld, 1);
nir_ssa_def *div = nir_bcsel(bld, tmp1,
nir_iadd(bld, quotient, one), quotient);
return nir_bcsel(bld, remainder_ge_zero,
div, nir_isub(bld, quotient, one));
return nir_bcsel(bld, remainder_ge_den, nir_iadd_imm(bld, quotient, 1),
quotient);
}
}