[llvm] [SECV] Try to push the op into ZExt: A + zext (-A + B) -> zext (B) (PR #151227)
Florian Hahn via llvm-commits
llvm-commits at lists.llvm.org
Wed Jul 30 02:05:32 PDT 2025
https://github.com/fhahn updated https://github.com/llvm/llvm-project/pull/151227
>From c37ee78b77e8897bce02c5b2600594c105ba2406 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Tue, 29 Jul 2025 16:24:17 +0100
Subject: [PATCH 1/2] [SECV] Try to push the op into ZExt: A + zext (-A + B) ->
zext (B)
Try to push the constant operand into a ZExt:
A + zext (-A + B) -> zext (B), if trunc (A) + -A + B does not unsigned-wrap.
The actual code supports ZExts with arbitrary number of arguments, hence
the getAddExpr in the return.
This helps SCEV reasoning in some cases, commonly when adding an offset
to a zero-extended SCEV that subtracts the same offset.
Note that this is restricted to cases where we can fold away an operand
of the inner Add. This is needed to avoid bad interactions with patterns
when forming ZExts, which try to push to ZExt to add operands.
https://alive2.llvm.org/ce/z/uuYGC3k
---
llvm/lib/Analysis/ScalarEvolution.cpp | 15 +++++++++++++++
.../IndVarSimplify/AArch64/fold-ext-add.ll | 10 +++++-----
llvm/test/Transforms/IndVarSimplify/zext-nuw.ll | 6 ++----
3 files changed, 22 insertions(+), 9 deletions(-)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 0990a0daac80c..8e587d8cbced6 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2682,6 +2682,21 @@ const SCEV *ScalarEvolution::getAddExpr(SmallVectorImpl<const SCEV *> &Ops,
return getAddExpr(NewOps, PreservedFlags);
}
}
+
+ // Try to push the constant operand into a ZExt: A + zext (-A + B) -> zext
+ // (B), if trunc (A) + -A + B does not unsigned-wrap.
+ if (auto *ZExt = dyn_cast<SCEVZeroExtendExpr>(Ops[1])) {
+ const SCEV *B = ZExt->getOperand(0);
+ const SCEV *NarrowA = getTruncateExpr(A, B->getType());
+ if (isa<SCEVAddExpr>(B) &&
+ NarrowA == getNegativeSCEV(cast<SCEVAddExpr>(B)->getOperand(0)) &&
+ getZeroExtendExpr(NarrowA, ZExt->getType()) == A &&
+ hasFlags(
+ StrengthenNoWrapFlags(this, scAddExpr, {NarrowA, B}, OrigFlags),
+ SCEV::FlagNUW)) {
+ return getZeroExtendExpr(getAddExpr(NarrowA, B), ZExt->getType());
+ }
+ }
}
// Canonicalize (-1 * urem X, Y) + X --> (Y * X/Y)
diff --git a/llvm/test/Transforms/IndVarSimplify/AArch64/fold-ext-add.ll b/llvm/test/Transforms/IndVarSimplify/AArch64/fold-ext-add.ll
index 48b92e905b8ce..640c910be4a82 100644
--- a/llvm/test/Transforms/IndVarSimplify/AArch64/fold-ext-add.ll
+++ b/llvm/test/Transforms/IndVarSimplify/AArch64/fold-ext-add.ll
@@ -10,21 +10,21 @@ define void @pred_mip_12(ptr %dst, ptr %src, i32 %n, i64 %offset) {
; CHECK-SAME: ptr [[DST:%.*]], ptr [[SRC:%.*]], i32 [[N:%.*]], i64 [[OFFSET:%.*]]) {
; CHECK-NEXT: [[ENTRY:.*]]:
; CHECK-NEXT: [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[N]], i32 1)
+; CHECK-NEXT: [[TMP0:%.*]] = zext nneg i32 [[SMAX]] to i64
+; CHECK-NEXT: [[TMP1:%.*]] = mul i64 [[OFFSET]], [[TMP0]]
+; CHECK-NEXT: [[SCEVGEP:%.*]] = getelementptr i8, ptr [[SRC]], i64 [[TMP1]]
; CHECK-NEXT: br label %[[OUTER_LOOP:.*]]
; CHECK: [[OUTER_LOOP_LOOPEXIT:.*]]:
-; CHECK-NEXT: [[PTR_IV_NEXT_LCSSA:%.*]] = phi ptr [ [[PTR_IV_NEXT:%.*]], %[[INNER_LOOP:.*]] ]
; CHECK-NEXT: br label %[[OUTER_LOOP]]
; CHECK: [[OUTER_LOOP]]:
-; CHECK-NEXT: [[OUTER_PTR:%.*]] = phi ptr [ [[SRC]], %[[ENTRY]] ], [ [[PTR_IV_NEXT_LCSSA]], %[[OUTER_LOOP_LOOPEXIT]] ]
+; CHECK-NEXT: [[OUTER_PTR:%.*]] = phi ptr [ [[SRC]], %[[ENTRY]] ], [ [[SCEVGEP]], %[[OUTER_LOOP_LOOPEXIT]] ]
; CHECK-NEXT: [[C:%.*]] = call i1 @cond()
; CHECK-NEXT: br i1 [[C]], label %[[INNER_LOOP_PREHEADER:.*]], label %[[EXIT:.*]]
; CHECK: [[INNER_LOOP_PREHEADER]]:
-; CHECK-NEXT: br label %[[INNER_LOOP]]
+; CHECK-NEXT: br label %[[INNER_LOOP:.*]]
; CHECK: [[INNER_LOOP]]:
; CHECK-NEXT: [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], %[[INNER_LOOP]] ], [ 0, %[[INNER_LOOP_PREHEADER]] ]
-; CHECK-NEXT: [[PTR_IV:%.*]] = phi ptr [ [[PTR_IV_NEXT]], %[[INNER_LOOP]] ], [ [[SRC]], %[[INNER_LOOP_PREHEADER]] ]
; CHECK-NEXT: [[L:%.*]] = load i8, ptr [[OUTER_PTR]], align 1
-; CHECK-NEXT: [[PTR_IV_NEXT]] = getelementptr i8, ptr [[PTR_IV]], i64 [[OFFSET]]
; CHECK-NEXT: store i8 [[L]], ptr [[DST]], align 2
; CHECK-NEXT: [[IV_NEXT]] = add nuw nsw i32 [[IV]], 1
; CHECK-NEXT: [[EXITCOND:%.*]] = icmp ne i32 [[IV_NEXT]], [[SMAX]]
diff --git a/llvm/test/Transforms/IndVarSimplify/zext-nuw.ll b/llvm/test/Transforms/IndVarSimplify/zext-nuw.ll
index d24f9a4e40e38..17921afc5ff06 100644
--- a/llvm/test/Transforms/IndVarSimplify/zext-nuw.ll
+++ b/llvm/test/Transforms/IndVarSimplify/zext-nuw.ll
@@ -15,11 +15,9 @@ define void @_Z3fn1v() {
; CHECK-NEXT: [[J_SROA_0_0_COPYLOAD:%.*]] = load i8, ptr [[X5]], align 1
; CHECK-NEXT: br label [[DOTPREHEADER4_LR_PH:%.*]]
; CHECK: .preheader4.lr.ph:
-; CHECK-NEXT: [[TMP1:%.*]] = add nsw i32 [[X4]], -1
-; CHECK-NEXT: [[TMP2:%.*]] = zext nneg i32 [[TMP1]] to i64
-; CHECK-NEXT: [[TMP3:%.*]] = add nuw nsw i64 [[TMP2]], 1
; CHECK-NEXT: [[TMP4:%.*]] = sext i8 [[J_SROA_0_0_COPYLOAD]] to i64
-; CHECK-NEXT: [[TMP5:%.*]] = mul i64 [[TMP3]], [[TMP4]]
+; CHECK-NEXT: [[TMP2:%.*]] = zext nneg i32 [[X4]] to i64
+; CHECK-NEXT: [[TMP5:%.*]] = mul i64 [[TMP4]], [[TMP2]]
; CHECK-NEXT: br label [[DOTPREHEADER4:%.*]]
; CHECK: .preheader4:
; CHECK-NEXT: [[K_09:%.*]] = phi ptr [ undef, [[DOTPREHEADER4_LR_PH]] ], [ [[X25:%.*]], [[X22:%.*]] ]
>From e834442b1563e34ee10e181a4f4a154ef0cb2660 Mon Sep 17 00:00:00 2001
From: Florian Hahn <flo at fhahn.com>
Date: Wed, 30 Jul 2025 09:53:04 +0100
Subject: [PATCH 2/2] !fixup pass FlagAnyWrap.
---
llvm/lib/Analysis/ScalarEvolution.cpp | 6 +++---
llvm/test/Analysis/ScalarEvolution/zext-add.ll | 2 +-
2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 8e587d8cbced6..61a575c8a0370 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -2691,9 +2691,9 @@ const SCEV *ScalarEvolution::getAddExpr(SmallVectorImpl<const SCEV *> &Ops,
if (isa<SCEVAddExpr>(B) &&
NarrowA == getNegativeSCEV(cast<SCEVAddExpr>(B)->getOperand(0)) &&
getZeroExtendExpr(NarrowA, ZExt->getType()) == A &&
- hasFlags(
- StrengthenNoWrapFlags(this, scAddExpr, {NarrowA, B}, OrigFlags),
- SCEV::FlagNUW)) {
+ hasFlags(StrengthenNoWrapFlags(this, scAddExpr, {NarrowA, B},
+ SCEV::FlagAnyWrap),
+ SCEV::FlagNUW)) {
return getZeroExtendExpr(getAddExpr(NarrowA, B), ZExt->getType());
}
}
diff --git a/llvm/test/Analysis/ScalarEvolution/zext-add.ll b/llvm/test/Analysis/ScalarEvolution/zext-add.ll
index 3290ee2deb4e8..a08feef7098ea 100644
--- a/llvm/test/Analysis/ScalarEvolution/zext-add.ll
+++ b/llvm/test/Analysis/ScalarEvolution/zext-add.ll
@@ -17,7 +17,7 @@ define void @test_push_constant_into_zext(ptr %dst, ptr %src, i32 %n, i64 %offse
; CHECK-NEXT: %l = load i8, ptr %outer.ptr, align 1
; CHECK-NEXT: --> %l U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %inner.loop: Variant, %outer.loop: Variant }
; CHECK-NEXT: %ptr.iv.next = getelementptr i8, ptr %ptr.iv, i64 %offset
-; CHECK-NEXT: --> {(%offset + %src),+,%offset}<%inner.loop> U: full-set S: full-set Exits: (((1 + (zext i32 (-1 + (1 smax %n))<nsw> to i64))<nuw><nsw> * %offset) + %src) LoopDispositions: { %inner.loop: Computable, %outer.loop: Variant }
+; CHECK-NEXT: --> {(%offset + %src),+,%offset}<%inner.loop> U: full-set S: full-set Exits: (((zext i32 (1 smax %n) to i64) * %offset) + %src) LoopDispositions: { %inner.loop: Computable, %outer.loop: Variant }
; CHECK-NEXT: %iv.next = add i32 %iv, 1
; CHECK-NEXT: --> {1,+,1}<nuw><nsw><%inner.loop> U: [1,-2147483648) S: [1,-2147483648) Exits: (1 smax %n) LoopDispositions: { %inner.loop: Computable, %outer.loop: Variant }
; CHECK-NEXT: Determining loop execution counts for: @test_push_constant_into_zext
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