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[DemandedBits] Add div/rem support #148853

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[DemandedBits] Add div/rem support #148853

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78db179
[LLVM] DemandedBits: Propagate demanded bits through div/rem ops
karouzakisp Jul 15, 2025
6d9b271
[LLVM] used APInt's API to be safe with from larger than 64 bits div/…
karouzakisp Jul 15, 2025
d2df527
minor updates and added new test that was supposed to be failing
karouzakisp Jul 16, 2025
e1c0bde
fixed Check-Label
karouzakisp Jul 16, 2025
179eb22
fixed srem, we need to preserve the sign bit, otherwise we might lose it
karouzakisp Jul 16, 2025
216f44b
removed risky demanded bits propagation with division
karouzakisp Jul 16, 2025
aada689
set sign bit on sdiv and srem to avoid missing the sign bit
karouzakisp Jul 16, 2025
64b366f
removed sdiv, udiv, urem
karouzakisp Jul 23, 2025
80d03bf
updated test
karouzakisp Jul 23, 2025
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14 changes: 14 additions & 0 deletions llvm/lib/Analysis/DemandedBits.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -246,6 +246,20 @@ void DemandedBits::determineLiveOperandBits(
else
AB &= ~(Known.One & ~Known2.One);
break;
case Instruction::SRem: {
// urem and udiv will be converted to and/lshr
// multiple times and early on. So, we don't
// need to calculate demanded-bits for those.
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@nikic nikic Jul 25, 2025

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This comment looks oddly narrow.

const APInt *DivAmnt;
if (match(UserI->getOperand(1), m_APInt(DivAmnt))) {
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@nikic nikic Jul 25, 2025

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Suggested change
if (match(UserI->getOperand(1), m_APInt(DivAmnt))) {
if (match(UserI->getOperand(1), m_Power2(DivAmnt))) {

Then you don't need the separate check.

if (DivAmnt->isPowerOf2()) {
unsigned Sh = DivAmnt->countr_zero();
AB = AOut & APInt::getLowBitsSet(BitWidth, Sh);
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@nikic nikic Jul 25, 2025

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Suggested change
AB = AOut & APInt::getLowBitsSet(BitWidth, Sh);
AB = AOut & (DivAmnt - 1);

A slightly simpler way to express this (matching InstCombineSimplifyDemanded).

AB.setSignBit();
}
}
break;
}
case Instruction::Xor:
case Instruction::PHI:
AB = AOut;
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83 changes: 83 additions & 0 deletions llvm/test/Analysis/DemandedBits/rem.ll
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@nikic nikic Jul 25, 2025

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I think none of the tests currently show the influence of the AOut demanded bits on AB. If you're using trunc i8, you need to test a pow2 >= 512 for that.

Original file line number Diff line number Diff line change
@@ -0,0 +1,83 @@
; RUN: opt -S -disable-output -passes="print<demanded-bits>" < %s 2>&1 | FileCheck %s


define i8 @test_srem_zext_trunc_const_amount2(i8 %a) {
; CHECK-LABEL: 'test_srem_zext_trunc_const_amount2'
; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %ext, 2
; CHECK-DAG: DemandedBits: 0x80000001 for %ext in %rem = srem i32 %ext, 2
; CHECK-DAG: DemandedBits: 0x80000001 for 2 in %rem = srem i32 %ext, 2
; CHECK-DAG: DemandedBits: 0xff for %rem in %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0x80000001 for %ext = sext i8 %a to i32
; CHECK-DAG: DemandedBits: 0x81 for %a in %ext = sext i8 %a to i32
;
%ext = sext i8 %a to i32
%rem = srem i32 %ext, 2
%rem.t = trunc i32 %rem to i8
ret i8 %rem.t
}

define i8 @test_srem_const_amount_4(i32 %a) {
; CHECK-LABEL: 'test_srem_const_amount_4'
; CHECK-DAG: DemandedBits: 0xff for %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem in %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %a, 4
; CHECK-DAG: DemandedBits: 0x80000003 for %a in %rem = srem i32 %a, 4
; CHECK-DAG: DemandedBits: 0x80000003 for 4 in %rem = srem i32 %a, 4
;
%rem = srem i32 %a, 4
%rem.t = trunc i32 %rem to i8
ret i8 %rem.t
}

define i8 @test_srem_const_amount_5(i32 %a) {
; CHECK-LABEL: 'test_srem_const_amount_5'
; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %a, 5
; CHECK-DAG: DemandedBits: 0xffffffff for %a in %rem = srem i32 %a, 5
; CHECK-DAG: DemandedBits: 0xffffffff for 5 in %rem = srem i32 %a, 5
; CHECK-DAG: DemandedBits: 0xff for %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem in %rem.t = trunc i32 %rem to i8
;
%rem = srem i32 %a, 5
%rem.t = trunc i32 %rem to i8
ret i8 %rem.t
}

define i8 @test_srem_const_amount_8(i32 %a) {
; CHECK-LABEL: 'test_srem_const_amount_8'
; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %a, 8
; CHECK-DAG: DemandedBits: 0x80000007 for %a in %rem = srem i32 %a, 8
; CHECK-DAG: DemandedBits: 0x80000007 for 8 in %rem = srem i32 %a, 8
; CHECK-DAG: DemandedBits: 0xff for %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem in %rem.t = trunc i32 %rem to i8
;
%rem = srem i32 %a, 8
%rem.t = trunc i32 %rem to i8
ret i8 %rem.t
}

define i8 @test_srem_const_amount_9(i32 %a) {
; CHECK-LABEL: 'test_srem_const_amount_9'
; CHECK-DAG: DemandedBits: 0xff for %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem in %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %a, 9
; CHECK-DAG: DemandedBits: 0xffffffff for %a in %rem = srem i32 %a, 9
; CHECK-DAG: DemandedBits: 0xffffffff for 9 in %rem = srem i32 %a, 9
;
%rem = srem i32 %a, 9
%rem.t = trunc i32 %rem to i8
ret i8 %rem.t
}

define i8 @test_srem(i32 %a, i32 %b) {
; CHECK-LABEL: 'test_srem'
; CHECK-DAG: DemandedBits: 0xff for %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem in %rem.t = trunc i32 %rem to i8
; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %a, %b
; CHECK-DAG: DemandedBits: 0xffffffff for %a in %rem = srem i32 %a, %b
; CHECK-DAG: DemandedBits: 0xffffffff for %b in %rem = srem i32 %a, %b
;
%rem = srem i32 %a, %b
%rem.t = trunc i32 %rem to i8
ret i8 %rem.t
}