[llvm] [GlobalISel] Add G_BITCAST to computeKnownBits (PR #217841)
Deepak Shirke via llvm-commits
llvm-commits at lists.llvm.org
Fri Aug 21 01:17:01 PDT 2026
https://github.com/deepakshirkem created https://github.com/llvm/llvm-project/pull/217841
Port the SDAG BITCAST computeKnownBits handling to GlobalISel.
- Identity bitcast (same bit width): propagate known bits directly
- Small element vector to large element scalar/vector: combine known bits from sub-elements
- Large element scalar/vector to small element vector: extract known bits per element
Part of #150515.
>From d532587dc4611edc1b7225488df45657cf0440cf Mon Sep 17 00:00:00 2001
From: deepakshirkem <deepakshirke509 at gmail.com>
Date: Fri, 21 Aug 2026 13:43:22 +0530
Subject: [PATCH] [GlobalISel] Add G_BITCAST to computeKnownBits
Port the SDAG BITCAST computeKnownBits handling to GlobalISel.
Handles three cases:
- Identity bitcast (same bit width): propagate known bits directly
- Small element vector to large element scalar/vector: combine known
bits from sub-elements using insertBits
- Large element scalar/vector to small element vector: extract known
bits per element using extractBits
---
.../CodeGen/GlobalISel/GISelValueTracking.cpp | 62 +++++++++++++++++++
.../AArch64/GlobalISel/knownbits-bitcast.mir | 57 +++++++++++++++++
2 files changed, 119 insertions(+)
create mode 100644 llvm/test/CodeGen/AArch64/GlobalISel/knownbits-bitcast.mir
diff --git a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
index 0672cfffc6eac..132606499bdd7 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
@@ -879,6 +879,68 @@ void GISelValueTracking::computeKnownBitsImpl(Register R, KnownBits &Known,
Known = Known.reverseBits();
break;
}
+ case TargetOpcode::G_BITCAST: {
+ Register SrcReg = MI.getOperand(1).getReg();
+ LLT SrcTy = MRI.getType(SrcReg);
+
+ // Ignore bitcasts involving scalable vectors.
+ if (SrcTy.isScalableVector() || DstTy.isScalableVector())
+ break;
+
+ unsigned SrcBitWidth = SrcTy.getScalarSizeInBits();
+
+ // Ignore bitcasts from unsupported types.
+ if (!SrcTy.isInteger() && !SrcTy.isFloat() && !SrcTy.isIntegerVector() &&
+ !SrcTy.isFloatVector())
+ break;
+
+ // Fast handling of identity bitcasts (same bit width).
+ if (BitWidth == SrcBitWidth) {
+ computeKnownBitsImpl(SrcReg, Known, DemandedElts, Depth + 1);
+ break;
+ }
+
+ bool IsLE = DL.isLittleEndian();
+ unsigned NumElts = DstTy.isVector() ? DstTy.getNumElements() : 1;
+
+ // Bitcast 'small element' vector to 'large element' scalar/vector.
+ if ((BitWidth % SrcBitWidth) == 0) {
+ assert(SrcTy.isVector() && "Expected bitcast from vector");
+ unsigned SubScale = BitWidth / SrcBitWidth;
+ unsigned NumSrcElts = SrcTy.getNumElements();
+ APInt SubDemandedElts(NumSrcElts, 0);
+ for (unsigned i = 0; i != NumElts; ++i)
+ if (DemandedElts[i])
+ SubDemandedElts.setBit(i * SubScale);
+ for (unsigned i = 0; i != SubScale; ++i) {
+ KnownBits SubKnown(SrcBitWidth);
+ computeKnownBitsImpl(SrcReg, SubKnown, SubDemandedElts.shl(i),
+ Depth + 1);
+ unsigned Shifts = IsLE ? i : SubScale - 1 - i;
+ Known.insertBits(SubKnown, SrcBitWidth * Shifts);
+ }
+ }
+
+ // Bitcast 'large element' scalar/vector to 'small element' vector.
+ if ((SrcBitWidth % BitWidth) == 0) {
+ assert(DstTy.isVector() && "Expected bitcast to vector");
+ unsigned SubScale = SrcBitWidth / BitWidth;
+ APInt SubDemandedElts =
+ APIntOps::ScaleBitMask(DemandedElts, NumElts / SubScale);
+ // Use SrcBitWidth for Known2 since source is wider than destination
+ KnownBits SrcKnown(SrcBitWidth);
+ computeKnownBitsImpl(SrcReg, SrcKnown, SubDemandedElts, Depth + 1);
+ Known.setAllConflict();
+ for (unsigned i = 0; i != NumElts; ++i) {
+ if (DemandedElts[i]) {
+ unsigned Shifts = IsLE ? i : NumElts - 1 - i;
+ unsigned Offset = (Shifts % SubScale) * BitWidth;
+ Known = Known.intersectWith(SrcKnown.extractBits(BitWidth, Offset));
+ }
+ }
+ }
+ break;
+ }
case TargetOpcode::G_CTPOP: {
computeKnownBitsImpl(MI.getOperand(1).getReg(), Known2, DemandedElts,
Depth + 1);
diff --git a/llvm/test/CodeGen/AArch64/GlobalISel/knownbits-bitcast.mir b/llvm/test/CodeGen/AArch64/GlobalISel/knownbits-bitcast.mir
new file mode 100644
index 0000000000000..f78c4120e1987
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/GlobalISel/knownbits-bitcast.mir
@@ -0,0 +1,57 @@
+# NOTE: Assertions have been autogenerated by utils/update_givaluetracking_test_checks.py UTC_ARGS: --version 6
+# RUN: llc -mtriple aarch64 -passes="print<gisel-value-tracking>" %s -filetype=null 2>&1 | FileCheck %s
+---
+# Identity bitcast i32->f32->i32, known bits preserved
+name: Bitcast_Identity
+body: |
+ bb.0:
+ ; CHECK-LABEL: name: @Bitcast_Identity
+ ; CHECK-NEXT: %0:_ KnownBits:00000000000000000000000000001111 SignBits:28 IsKnownNeverZero:1
+ ; CHECK-NEXT: %1:_ KnownBits:00000000000000000000000000001111 SignBits:28 IsKnownNeverZero:1
+ ; CHECK-NEXT: %2:_ KnownBits:00000000000000000000000000001111 SignBits:28 IsKnownNeverZero:1
+ %0:_(i32) = G_CONSTANT i32 15
+ %1:_(f32) = G_BITCAST %0
+ %2:_(i32) = G_BITCAST %1
+...
+---
+# Large->Small: i32 with known zero high 16 bits -> <2 x i16>
+# Element 1 (high i16) should be known zero
+# Verified via G_EXTRACT_VECTOR_ELT which demands only element 1
+name: Bitcast_LargeToSmall_HighElt_KnownZero
+body: |
+ bb.0:
+ ; CHECK-LABEL: name: @Bitcast_LargeToSmall_HighElt_KnownZero
+ ; CHECK-NEXT: %0:_ KnownBits:???????????????????????????????? SignBits:1 IsKnownNeverZero:0
+ ; CHECK-NEXT: %1:_ KnownBits:00000000000000001111111111111111 SignBits:16 IsKnownNeverZero:1
+ ; CHECK-NEXT: %2:_ KnownBits:0000000000000000???????????????? SignBits:16 IsKnownNeverZero:0
+ ; CHECK-NEXT: %3:_ KnownBits:???????????????? SignBits:1 IsKnownNeverZero:0
+ ; CHECK-NEXT: %4:_ KnownBits:0000000000000000000000000000000000000000000000000000000000000001 SignBits:63 IsKnownNeverZero:1
+ ; CHECK-NEXT: %5:_ KnownBits:0000000000000000 SignBits:16 IsKnownNeverZero:0
+ %0:_(i32) = COPY $w0
+ %1:_(i32) = G_CONSTANT i32 65535
+ %2:_(i32) = G_AND %0, %1
+ %3:_(<2 x i16>) = G_BITCAST %2
+ %4:_(i64) = G_CONSTANT i64 1
+ %5:_(i16) = G_EXTRACT_VECTOR_ELT %3, %4
+...
+---
+# Small->Large: <2 x i16> with known zero high bytes -> i32
+name: Bitcast_SmallToLarge_KnownZero
+body: |
+ bb.0:
+ ; CHECK-LABEL: name: @Bitcast_SmallToLarge_KnownZero
+ ; CHECK-NEXT: %0:_ KnownBits:???????????????? SignBits:1 IsKnownNeverZero:0
+ ; CHECK-NEXT: %1:_ KnownBits:0000000011111111 SignBits:8 IsKnownNeverZero:1
+ ; CHECK-NEXT: %2:_ KnownBits:00000000???????? SignBits:8 IsKnownNeverZero:0
+ ; CHECK-NEXT: %3:_ KnownBits:???????????????? SignBits:1 IsKnownNeverZero:0
+ ; CHECK-NEXT: %4:_ KnownBits:00000000???????? SignBits:8 IsKnownNeverZero:0
+ ; CHECK-NEXT: %5:_ KnownBits:00000000???????? SignBits:8 IsKnownNeverZero:0
+ ; CHECK-NEXT: %6:_ KnownBits:00000000????????00000000???????? SignBits:8 IsKnownNeverZero:0
+ %0:_(i16) = COPY $h0
+ %1:_(i16) = G_CONSTANT i16 255
+ %2:_(i16) = G_AND %0, %1
+ %3:_(i16) = COPY $h1
+ %4:_(i16) = G_AND %3, %1
+ %5:_(<2 x i16>) = G_BUILD_VECTOR %2, %4
+ %6:_(i32) = G_BITCAST %5
+...
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