[llvm] [Loads] Support dereference for non-constant offset (PR #149551)
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llvm-commits at lists.llvm.org
Thu Aug 7 12:42:40 PDT 2025
https://github.com/annamthomas updated https://github.com/llvm/llvm-project/pull/149551
>From 23ac9f05b01a20a62b8d12253afb51fb3c591950 Mon Sep 17 00:00:00 2001
From: Anna Thomas <anna at azul.com>
Date: Thu, 17 Jul 2025 17:48:13 -0400
Subject: [PATCH 1/2] [Loads] Support dereference for non-constant offset
Improve isDereferenceableAndAlignedInLoop API to consider non-constant
offset feeding into the first access in the loop.
---
llvm/lib/Analysis/Loads.cpp | 28 ++++++-------
...able-info-from-assumption-variable-size.ll | 42 +++++--------------
2 files changed, 25 insertions(+), 45 deletions(-)
diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index 78d0887d5d87e..250b46c6560eb 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -361,29 +361,29 @@ bool llvm::isDereferenceableAndAlignedInLoop(
AccessSize = MaxPtrDiff;
AccessSizeSCEV = PtrDiff;
} else if (auto *MinAdd = dyn_cast<SCEVAddExpr>(AccessStart)) {
- if (MinAdd->getNumOperands() != 2)
- return false;
+ const auto *NewBase = dyn_cast<SCEVUnknown>(SE.getPointerBase(MinAdd));
+ const auto *OffsetSCEV = SE.removePointerBase(MinAdd);
- const auto *Offset = dyn_cast<SCEVConstant>(MinAdd->getOperand(0));
- const auto *NewBase = dyn_cast<SCEVUnknown>(MinAdd->getOperand(1));
- if (!Offset || !NewBase)
+ if (!OffsetSCEV || !NewBase)
return false;
- // The following code below assumes the offset is unsigned, but GEP
- // offsets are treated as signed so we can end up with a signed value
- // here too. For example, suppose the initial PHI value is (i8 255),
- // the offset will be treated as (i8 -1) and sign-extended to (i64 -1).
- if (Offset->getAPInt().isNegative())
+ if (!SE.isKnownNonNegative(OffsetSCEV))
return false;
// For the moment, restrict ourselves to the case where the offset is a
// multiple of the requested alignment and the base is aligned.
// TODO: generalize if a case found which warrants
- if (Offset->getAPInt().urem(Alignment.value()) != 0)
+ auto *OffsetSCEVTy = OffsetSCEV->getType();
+ if (!SE.isKnownPredicate(
+ ICmpInst::ICMP_EQ,
+ SE.getURemExpr(OffsetSCEV,
+ SE.getConstant(OffsetSCEVTy, Alignment.value())),
+ SE.getZero(OffsetSCEVTy)))
return false;
-
- AccessSize = MaxPtrDiff + Offset->getAPInt();
- AccessSizeSCEV = SE.getAddExpr(PtrDiff, Offset);
+ AccessSizeSCEV = SE.getAddExpr(PtrDiff, OffsetSCEV);
+ const auto *Offset = dyn_cast<SCEVConstant>(OffsetSCEV);
+ AccessSize = MaxPtrDiff + (Offset ? Offset->getAPInt()
+ : SE.getUnsignedRangeMax(OffsetSCEV));
Base = NewBase->getValue();
} else
return false;
diff --git a/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-variable-size.ll b/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-variable-size.ll
index 9852f538c6f74..4320d7d90bf47 100644
--- a/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-variable-size.ll
+++ b/llvm/test/Transforms/LoopVectorize/dereferenceable-info-from-assumption-variable-size.ll
@@ -562,38 +562,19 @@ define void @deref_assumption_loop_access_start_variable(i8 %v, ptr noundef %P,
; CHECK-NEXT: [[TMP1:%.*]] = add i64 [[IV_START]], [[N_VEC]]
; CHECK-NEXT: br label %[[VECTOR_BODY:.*]]
; CHECK: [[VECTOR_BODY]]:
-; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[PRED_LOAD_CONTINUE2:.*]] ]
+; CHECK-NEXT: [[INDEX:%.*]] = phi i64 [ 0, %[[VECTOR_PH]] ], [ [[INDEX_NEXT:%.*]], %[[VECTOR_BODY]] ]
; CHECK-NEXT: [[OFFSET_IDX:%.*]] = add i64 [[IV_START]], [[INDEX]]
-; CHECK-NEXT: [[TMP6:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[OFFSET_IDX]]
-; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <2 x i32>, ptr [[TMP6]], align 1
-; CHECK-NEXT: [[TMP8:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD]], zeroinitializer
-; CHECK-NEXT: [[TMP4:%.*]] = xor <2 x i1> [[TMP8]], splat (i1 true)
-; CHECK-NEXT: [[TMP5:%.*]] = extractelement <2 x i1> [[TMP4]], i32 0
-; CHECK-NEXT: br i1 [[TMP5]], label %[[PRED_LOAD_IF:.*]], label %[[PRED_LOAD_CONTINUE:.*]]
-; CHECK: [[PRED_LOAD_IF]]:
-; CHECK-NEXT: [[TMP16:%.*]] = add i64 [[OFFSET_IDX]], 0
-; CHECK-NEXT: [[TMP7:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP16]]
-; CHECK-NEXT: [[TMP19:%.*]] = load i32, ptr [[TMP7]], align 1
-; CHECK-NEXT: [[TMP9:%.*]] = insertelement <2 x i32> poison, i32 [[TMP19]], i32 0
-; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE]]
-; CHECK: [[PRED_LOAD_CONTINUE]]:
-; CHECK-NEXT: [[TMP10:%.*]] = phi <2 x i32> [ poison, %[[VECTOR_BODY]] ], [ [[TMP9]], %[[PRED_LOAD_IF]] ]
-; CHECK-NEXT: [[TMP11:%.*]] = extractelement <2 x i1> [[TMP4]], i32 1
-; CHECK-NEXT: br i1 [[TMP11]], label %[[PRED_LOAD_IF1:.*]], label %[[PRED_LOAD_CONTINUE2]]
-; CHECK: [[PRED_LOAD_IF1]]:
-; CHECK-NEXT: [[TMP12:%.*]] = add i64 [[OFFSET_IDX]], 1
-; CHECK-NEXT: [[TMP13:%.*]] = getelementptr inbounds i32, ptr [[A]], i64 [[TMP12]]
-; CHECK-NEXT: [[TMP14:%.*]] = load i32, ptr [[TMP13]], align 1
-; CHECK-NEXT: [[TMP15:%.*]] = insertelement <2 x i32> [[TMP10]], i32 [[TMP14]], i32 1
-; CHECK-NEXT: br label %[[PRED_LOAD_CONTINUE2]]
-; CHECK: [[PRED_LOAD_CONTINUE2]]:
-; CHECK-NEXT: [[WIDE_LOAD1:%.*]] = phi <2 x i32> [ [[TMP10]], %[[PRED_LOAD_CONTINUE]] ], [ [[TMP15]], %[[PRED_LOAD_IF1]] ]
-; CHECK-NEXT: [[PREDPHI:%.*]] = select <2 x i1> [[TMP8]], <2 x i32> [[WIDE_LOAD]], <2 x i32> [[WIDE_LOAD1]]
-; CHECK-NEXT: [[TMP17:%.*]] = getelementptr inbounds i32, ptr [[C]], i64 [[OFFSET_IDX]]
-; CHECK-NEXT: store <2 x i32> [[PREDPHI]], ptr [[TMP17]], align 1
+; CHECK-NEXT: [[TMP2:%.*]] = getelementptr i32, ptr [[A]], i64 [[OFFSET_IDX]]
+; CHECK-NEXT: [[TMP7:%.*]] = getelementptr inbounds i32, ptr [[B]], i64 [[OFFSET_IDX]]
+; CHECK-NEXT: [[WIDE_LOAD:%.*]] = load <2 x i32>, ptr [[TMP7]], align 1
+; CHECK-NEXT: [[TMP4:%.*]] = icmp sge <2 x i32> [[WIDE_LOAD]], zeroinitializer
+; CHECK-NEXT: [[WIDE_LOAD1:%.*]] = load <2 x i32>, ptr [[TMP2]], align 1
+; CHECK-NEXT: [[PREDPHI:%.*]] = select <2 x i1> [[TMP4]], <2 x i32> [[WIDE_LOAD]], <2 x i32> [[WIDE_LOAD1]]
+; CHECK-NEXT: [[TMP5:%.*]] = getelementptr inbounds i32, ptr [[C]], i64 [[OFFSET_IDX]]
+; CHECK-NEXT: store <2 x i32> [[PREDPHI]], ptr [[TMP5]], align 1
; CHECK-NEXT: [[INDEX_NEXT]] = add nuw i64 [[INDEX]], 2
-; CHECK-NEXT: [[TMP18:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
-; CHECK-NEXT: br i1 [[TMP18]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP14:![0-9]+]]
+; CHECK-NEXT: [[TMP6:%.*]] = icmp eq i64 [[INDEX_NEXT]], [[N_VEC]]
+; CHECK-NEXT: br i1 [[TMP6]], label %[[MIDDLE_BLOCK:.*]], label %[[VECTOR_BODY]], !llvm.loop [[LOOP14:![0-9]+]]
; CHECK: [[MIDDLE_BLOCK]]:
; CHECK-NEXT: [[CMP_N:%.*]] = icmp eq i64 [[TMP3]], [[N_VEC]]
; CHECK-NEXT: br i1 [[CMP_N]], label %[[EXIT:.*]], label %[[SCALAR_PH]]
@@ -620,7 +601,6 @@ define void @deref_assumption_loop_access_start_variable(i8 %v, ptr noundef %P,
; CHECK: [[EXIT]]:
; CHECK-NEXT: ret void
;
-
entry:
%a = getelementptr i8, ptr %P, i64 16
%cmp = icmp slt i64 %iv.start, %N
>From 2887ce4339541de9753f33cc8ba78f5b3da3e395 Mon Sep 17 00:00:00 2001
From: Anna Thomas <anna at azul.com>
Date: Thu, 7 Aug 2025 15:42:10 -0400
Subject: [PATCH 2/2] Address review comments
---
llvm/lib/Analysis/Loads.cpp | 16 ++++++----------
1 file changed, 6 insertions(+), 10 deletions(-)
diff --git a/llvm/lib/Analysis/Loads.cpp b/llvm/lib/Analysis/Loads.cpp
index 250b46c6560eb..42966f8cb153e 100644
--- a/llvm/lib/Analysis/Loads.cpp
+++ b/llvm/lib/Analysis/Loads.cpp
@@ -362,9 +362,9 @@ bool llvm::isDereferenceableAndAlignedInLoop(
AccessSizeSCEV = PtrDiff;
} else if (auto *MinAdd = dyn_cast<SCEVAddExpr>(AccessStart)) {
const auto *NewBase = dyn_cast<SCEVUnknown>(SE.getPointerBase(MinAdd));
- const auto *OffsetSCEV = SE.removePointerBase(MinAdd);
+ auto *OffsetSCEV = SE.removePointerBase(MinAdd);
- if (!OffsetSCEV || !NewBase)
+ if (!NewBase)
return false;
if (!SE.isKnownNonNegative(OffsetSCEV))
@@ -374,16 +374,12 @@ bool llvm::isDereferenceableAndAlignedInLoop(
// multiple of the requested alignment and the base is aligned.
// TODO: generalize if a case found which warrants
auto *OffsetSCEVTy = OffsetSCEV->getType();
- if (!SE.isKnownPredicate(
- ICmpInst::ICMP_EQ,
- SE.getURemExpr(OffsetSCEV,
- SE.getConstant(OffsetSCEVTy, Alignment.value())),
- SE.getZero(OffsetSCEVTy)))
+ if (!SE.getConstantMultiple(OffsetSCEV)
+ .urem(APInt(OffsetSCEVTy->getIntegerBitWidth(), Alignment.value()))
+ .isZero())
return false;
+ AccessSize = MaxPtrDiff + SE.getUnsignedRangeMax(OffsetSCEV);
AccessSizeSCEV = SE.getAddExpr(PtrDiff, OffsetSCEV);
- const auto *Offset = dyn_cast<SCEVConstant>(OffsetSCEV);
- AccessSize = MaxPtrDiff + (Offset ? Offset->getAPInt()
- : SE.getUnsignedRangeMax(OffsetSCEV));
Base = NewBase->getValue();
} else
return false;
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