[llvm] [LLVM] Improve the DemandedBits Analysis (PR #148853)
Panagiotis K via llvm-commits
llvm-commits at lists.llvm.org
Tue Jul 15 08:04:50 PDT 2025
https://github.com/karouzakisp updated https://github.com/llvm/llvm-project/pull/148853
>From fdae5ad5181b3ef1d601846542d4c434eeaf4039 Mon Sep 17 00:00:00 2001
From: Panagiotis Karouzakis <karouzakispan at gmail.com>
Date: Tue, 15 Jul 2025 18:01:29 +0300
Subject: [PATCH] [LLVM] DemandedBits: Propagate demanded bits through div/rem
ops
---
llvm/lib/Analysis/DemandedBits.cpp | 58 +++++
llvm/test/Analysis/DemandedBits/div_rem.ll | 261 +++++++++++++++++++++
2 files changed, 319 insertions(+)
create mode 100644 llvm/test/Analysis/DemandedBits/div_rem.ll
diff --git a/llvm/lib/Analysis/DemandedBits.cpp b/llvm/lib/Analysis/DemandedBits.cpp
index 6694d5cc06c8c..e5e82023cbc20 100644
--- a/llvm/lib/Analysis/DemandedBits.cpp
+++ b/llvm/lib/Analysis/DemandedBits.cpp
@@ -36,6 +36,7 @@
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/KnownBits.h"
+#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cstdint>
@@ -246,6 +247,63 @@ void DemandedBits::determineLiveOperandBits(
else
AB &= ~(Known.One & ~Known2.One);
break;
+ case Instruction::UDiv:
+ case Instruction::URem:
+ case Instruction::SDiv:
+ case Instruction::SRem: {
+ auto Opc = UserI->getOpcode();
+ auto IsDiv = Opc == Instruction::UDiv || Opc == Instruction::SDiv;
+ bool IsSigned = Opc == Instruction::SDiv || Opc == Instruction::SRem;
+ if (OperandNo == 0) {
+ const APInt *DivAmnt;
+ if (match(UserI->getOperand(1), m_APInt(DivAmnt))) {
+ uint64_t D = DivAmnt->getZExtValue();
+ if (isPowerOf2_64(D)) {
+ unsigned Sh = Log2_64(D);
+ if (IsDiv) {
+ AB = AOut.shl(Sh);
+ } else {
+ AB = AOut & APInt::getLowBitsSet(BitWidth, Sh);
+ }
+ } else { // Non power of 2 constant div
+ /*
+ * x = q * C + r;
+ * q = x / C;
+ * We think of it like grade school division in base 2.
+ *
+ x = [ unused | window m-bits | ... | needed bits ]
+ ^ each step emits 1 quotient bit
+ |
+ |
+ C fits in m = ⌈log₂ C⌉ bits |
+ Each new quotient bit consumes the window of m low bits and
+ shifts one position left.
+
+ To produce the first LowQ quotient/rem bits we slide the window
+ LowQ times --> need at most LowQ + m low bits of the dividend.
+ Need = LowQ + Ceil(log2(C)) (+1 sign bit for sdiv/srem).
+ For example :
+ Assume x = b7 b6 b5 b4 b3 b2 b1 b0.
+ LowQ = 4, C = 5 and ceil(log_2(C)) = 3.
+ step 0: b2 b1 b0, produces quotient q[0].
+ step 1: b3 b2 b1, produces quotient q[1].
+ step 2: b4 b3 b2, produces quotient q[2].
+ step 3: b5 b4 b3, produces quotient q[3].
+ k = LowQ - 1;
+ TopIndex = k + m-1 = 3 + 2 = 5;
+ The dividend bits b5...b0 are enough we don't care for b6 and b7.
+ The same applies to Urem/SRem
+ * */
+ unsigned LowQ = AOut.getActiveBits();
+ unsigned Need = LowQ + Log2_64_Ceil(D);
+ if (IsSigned)
+ Need++;
+ AB = APInt::getLowBitsSet(BitWidth, std::min(BitWidth, Need));
+ }
+ }
+ }
+ break;
+ }
case Instruction::Xor:
case Instruction::PHI:
AB = AOut;
diff --git a/llvm/test/Analysis/DemandedBits/div_rem.ll b/llvm/test/Analysis/DemandedBits/div_rem.ll
new file mode 100644
index 0000000000000..818cba17dc1a6
--- /dev/null
+++ b/llvm/test/Analysis/DemandedBits/div_rem.ll
@@ -0,0 +1,261 @@
+; RUN: opt -S -disable-output -passes="print<demanded-bits>" < %s 2>&1 | FileCheck %s
+
+define i8 @test_sdiv_const_amount_4(i32 %a) {
+; CHECK-LABEL: 'test_sdiv_const_amount_4'
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div = sdiv i32 %a, 4
+; CHECK-DAG: DemandedBits: 0x3fc for %a in %div = sdiv i32 %a, 4
+; CHECK-DAG: DemandedBits: 0xffffffff for 4 in %div = sdiv i32 %a, 4
+;
+ %div = sdiv i32 %a, 4
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_sdiv_const_amount_5(i32 %a) {
+; CHECK-LABEL: 'test_sdiv_const_amount_5'
+; CHECK-DAG: DemandedBits: 0xff for %div = sdiv i32 %a, 5
+; CHECK-DAG: DemandedBits: 0xfff for %a in %div = sdiv i32 %a, 5
+; CHECK-DAG: DemandedBits: 0xffffffff for 5 in %div = sdiv i32 %a, 5
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = sdiv i32 %a, 5
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_sdiv_const_amount_8(i32 %a) {
+; CHECK-LABEL: 'test_sdiv_const_amount_8'
+; CHECK-DAG: DemandedBits: 0xff for %div = sdiv i32 %a, 8
+; CHECK-DAG: DemandedBits: 0x7f8 for %a in %div = sdiv i32 %a, 8
+; CHECK-DAG: DemandedBits: 0xffffffff for 8 in %div = sdiv i32 %a, 8
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = sdiv i32 %a, 8
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_sdiv_const_amount_9(i32 %a) {
+; CHECK-LABEL: 'test_sdiv_const_amount_9'
+; CHECK-DAG: DemandedBits: 0xff for %div = udiv i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xfff for %a in %div = udiv i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xffffffff for 9 in %div = udiv i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = udiv i32 %a, 9
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_sdiv(i32 %a, i32 %b) {
+; CHECK-LABEL: 'test_sdiv'
+; CHECK-DAG: DemandedBits: 0xff for %div = sdiv i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xffffffff for %a in %div = sdiv i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xffffffff for %b in %div = sdiv i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = sdiv i32 %a, %b
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_udiv_const_amount_4(i32 %a) {
+; CHECK-LABEL: 'test_udiv_const_amount_4'
+; CHECK-DAG: DemandedBits: 0xff for %div = udiv i32 %a, 4
+; CHECK-DAG: DemandedBits: 0x3fc for %a in %div = udiv i32 %a, 4
+; CHECK-DAG: DemandedBits: 0xffffffff for 4 in %div = udiv i32 %a, 4
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = udiv i32 %a, 4
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_udiv_const_amount_5(i32 %a) {
+; CHECK-LABEL: 'test_udiv_const_amount_5'
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div = udiv i32 %a, 5
+; CHECK-DAG: DemandedBits: 0x7ff for %a in %div = udiv i32 %a, 5
+; CHECK-DAG: DemandedBits: 0xffffffff for 5 in %div = udiv i32 %a, 5
+;
+ %div = udiv i32 %a, 5
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_udiv_const_amount_8(i32 %a) {
+; CHECK-LABEL: 'test_udiv_const_amount_8'
+; CHECK-DAG: DemandedBits: 0xff for %div = udiv i32 %a, 8
+; CHECK-DAG: DemandedBits: 0x7f8 for %a in %div = udiv i32 %a, 8
+; CHECK-DAG: DemandedBits: 0xffffffff for 8 in %div = udiv i32 %a, 8
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = udiv i32 %a, 8
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_udiv_const_amount_9(i32 %a) {
+; CHECK-LABEL: 'test_udiv_const_amount_9'
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div = udiv i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xfff for %a in %div = udiv i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xffffffff for 9 in %div = udiv i32 %a, 9
+;
+ %div = udiv i32 %a, 9
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_udiv(i32 %a, i32 %b) {
+; CHECK-LABEL: 'test_udiv'
+; CHECK-DAG: DemandedBits: 0xff for %div = udiv i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xffffffff for %a in %div = udiv i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xffffffff for %b in %div = udiv i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xff for %div.t = trunc i32 %div to i8
+; CHECK-DAG: DemandedBits: 0xff for %div in %div.t = trunc i32 %div to i8
+;
+ %div = udiv i32 %a, %b
+ %div.t = trunc i32 %div to i8
+ ret i8 %div.t
+}
+
+define i8 @test_srem_const_amount_4(i32 %a) {
+; CHECK-LABEL: 'test_srem_const_amount_4'
+; CHECK-DAG: DemandedBits: 0xff for %rem = srem i32 %a, 4
+; CHECK-DAG: DemandedBits: 0x3 for %a in %rem = srem i32 %a, 4
+; CHECK-DAG: DemandedBits: 0xffffffff for 4 in %rem = srem i32 %a, 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
+;
+ %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.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, 5
+; CHECK-DAG: DemandedBits: 0xfff for %a in %rem = srem i32 %a, 5
+; CHECK-DAG: DemandedBits: 0xffffffff for 5 in %rem = srem i32 %a, 5
+;
+ %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: 0x7 for %a in %rem = srem i32 %a, 8
+; CHECK-DAG: DemandedBits: 0xffffffff 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: 0x1fff 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 = 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
+; 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, %b
+ %rem.t = trunc i32 %rem to i8
+ ret i8 %rem.t
+}
+
+define i8 @test_urem_const_amount_4(i32 %a) {
+; CHECK-LABEL: 'test_urem_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 = urem i32 %a, 4
+; CHECK-DAG: DemandedBits: 0x3 for %a in %rem = urem i32 %a, 4
+; CHECK-DAG: DemandedBits: 0xffffffff for 4 in %rem = urem i32 %a, 4
+;
+ %rem = urem i32 %a, 4
+ %rem.t = trunc i32 %rem to i8
+ ret i8 %rem.t
+}
+
+define i8 @test_urem_const_amount_5(i32 %a) {
+; CHECK-LABEL: 'test_urem_const_amount_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
+; CHECK-DAG: DemandedBits: 0xff for %rem = urem i32 %a, 5
+; CHECK-DAG: DemandedBits: 0x7ff for %a in %rem = urem i32 %a, 5
+; CHECK-DAG: DemandedBits: 0xffffffff for 5 in %rem = urem i32 %a, 5
+;
+ %rem = urem i32 %a, 5
+ %rem.t = trunc i32 %rem to i8
+ ret i8 %rem.t
+}
+
+define i8 @test_urem_const_amount_8(i32 %a) {
+; CHECK-LABEL: 'test_urem_const_amount_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
+; CHECK-DAG: DemandedBits: 0xff for %rem = urem i32 %a, 8
+; CHECK-DAG: DemandedBits: 0x7 for %a in %rem = urem i32 %a, 8
+; CHECK-DAG: DemandedBits: 0xffffffff for 8 in %rem = urem i32 %a, 8
+;
+ %rem = urem i32 %a, 8
+ %rem.t = trunc i32 %rem to i8
+ ret i8 %rem.t
+}
+
+define i8 @test_urem_const_amount_9(i32 %a) {
+; CHECK-LABEL: 'test_urem_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 = urem i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xfff for %a in %rem = urem i32 %a, 9
+; CHECK-DAG: DemandedBits: 0xffffffff for 9 in %rem = urem i32 %a, 9
+;
+ %rem = urem i32 %a, 9
+ %rem.t = trunc i32 %rem to i8
+ ret i8 %rem.t
+}
+
+define i8 @test_urem(i32 %a, i32 %b) {
+; CHECK-LABEL: 'test_urem'
+; CHECK-DAG: DemandedBits: 0xff for %rem = urem i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xffffffff for %a in %rem = urem i32 %a, %b
+; CHECK-DAG: DemandedBits: 0xffffffff for %b in %rem = urem i32 %a, %b
+; 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 = urem i32 %a, %b
+ %rem.t = trunc i32 %rem to i8
+ ret i8 %rem.t
+}
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