[llvm] [SCEV] BECount to zero if `((-1 + (1 smax %x))<nsw> /u %x)` holds (PR #104580)
Antonio Frighetto via llvm-commits
llvm-commits at lists.llvm.org
Fri Aug 16 03:53:15 PDT 2024
https://github.com/antoniofrighetto created https://github.com/llvm/llvm-project/pull/104580
The SCEV expression `((-1 + (1 smax %x))<nsw> /u %x)` can be folded to zero for positive %x, and the add node is guaranteed not to wrap in the signed sense.
>From 9ca24c25ed4a2bd503c0e510a9c7a23f83c39044 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Fri, 16 Aug 2024 11:51:05 +0200
Subject: [PATCH 1/2] [SCEV] Introduce test for PR104580 (NFC)
---
.../udiv-of-x-xsmaxone-fold.ll | 78 +++++++++++++++++++
1 file changed, 78 insertions(+)
create mode 100644 llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll
diff --git a/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll b/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll
new file mode 100644
index 00000000000000..a9f6c1e62dfbc3
--- /dev/null
+++ b/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll
@@ -0,0 +1,78 @@
+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py
+; RUN: opt -disable-output "-passes=print<scalar-evolution>" < %s 2>&1 | FileCheck %s
+
+ at g_var = external local_unnamed_addr global i32, align 4
+
+define void @test(i32 noundef %x) {
+; CHECK-LABEL: 'test'
+; CHECK-NEXT: Classifying expressions for: @test
+; CHECK-NEXT: %g_var.promoted = load i32, ptr @g_var, align 4
+; CHECK-NEXT: --> %g_var.promoted U: full-set S: full-set
+; CHECK-NEXT: %smax = tail call i32 @llvm.smax.i32(i32 %x, i32 1)
+; CHECK-NEXT: --> (1 smax %x) U: [1,-2147483648) S: [1,-2147483648)
+; CHECK-NEXT: %0 = add nsw i32 %smax, -1
+; CHECK-NEXT: --> (-1 + (1 smax %x))<nsw> U: [0,2147483647) S: [0,2147483647)
+; CHECK-NEXT: %1 = udiv i32 %0, %x
+; CHECK-NEXT: --> ((-1 + (1 smax %x))<nsw> /u %x) U: [0,2147483647) S: [0,2147483647)
+; CHECK-NEXT: %2 = add nuw nsw i32 %1, 1
+; CHECK-NEXT: --> (1 + ((-1 + (1 smax %x))<nsw> /u %x))<nuw><nsw> U: [1,-2147483648) S: [1,-2147483648)
+;
+; CHECK: Determining loop execution counts for: @test
+; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+;
+entry:
+ %g_var.promoted = load i32, ptr @g_var, align 4
+ %smax = tail call i32 @llvm.smax.i32(i32 %x, i32 1)
+ %0 = add nsw i32 %smax, -1
+ %1 = udiv i32 %0, %x
+ %2 = add nuw nsw i32 %1, 1
+ %min.iters.check = icmp ult i32 %1, 7
+ br i1 %min.iters.check, label %for.body.preheader, label %vector.ph
+
+for.body.preheader:
+ %i.06.ph = phi i32 [ 0, %entry ], [ %ind.end, %middle.block ]
+ %add45.ph = phi i32 [ %g_var.promoted, %entry ], [ %7, %middle.block ]
+ br label %for.body
+
+vector.ph:
+ %n.vec = and i32 %2, -8
+ %ind.end = mul i32 %n.vec, %x
+ %3 = insertelement <4 x i32> <i32 poison, i32 0, i32 0, i32 0>, i32 %g_var.promoted, i64 0
+ %broadcast.splatinsert = insertelement <4 x i32> poison, i32 %x, i64 0
+ %broadcast.splat = shufflevector <4 x i32> %broadcast.splatinsert, <4 x i32> poison, <4 x i32> zeroinitializer
+ br label %vector.body
+
+vector.body:
+ %index = phi i32 [ 0, %vector.ph ], [ %index.next, %vector.body ]
+ %vec.phi = phi <4 x i32> [ %3, %vector.ph ], [ %4, %vector.body ]
+ %vec.phi7 = phi <4 x i32> [ zeroinitializer, %vector.ph ], [ %5, %vector.body ]
+ %4 = add <4 x i32> %vec.phi, %broadcast.splat
+ %5 = add <4 x i32> %vec.phi7, %broadcast.splat
+ %index.next = add nuw i32 %index, 8
+ %6 = icmp eq i32 %index.next, %n.vec
+ br i1 %6, label %middle.block, label %vector.body
+
+middle.block:
+ %bin.rdx = add <4 x i32> %5, %4
+ %7 = tail call i32 @llvm.vector.reduce.add.v4i32(<4 x i32> %bin.rdx)
+ %cmp.n = icmp eq i32 %2, %n.vec
+ br i1 %cmp.n, label %for.cond.cleanup, label %for.body.preheader
+
+for.cond.cleanup:
+ %add.lcssa = phi i32 [ %7, %middle.block ], [ %add, %for.body ]
+ store i32 %add.lcssa, ptr @g_var, align 4
+ ret void
+
+for.body:
+ %i.06 = phi i32 [ %add1, %for.body ], [ %i.06.ph, %for.body.preheader ]
+ %add45 = phi i32 [ %add, %for.body ], [ %add45.ph, %for.body.preheader ]
+ %add = add nsw i32 %add45, %x
+ %add1 = add nsw i32 %i.06, %x
+ %cmp = icmp slt i32 %add1, 1
+ br i1 %cmp, label %for.body, label %for.cond.cleanup
+}
+
+declare i32 @llvm.smax.i32(i32, i32)
+declare i32 @llvm.vector.reduce.add.v4i32(<4 x i32>)
>From f5415d86e626ebf0618c72fb95753556b43dd094 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Fri, 16 Aug 2024 12:44:20 +0200
Subject: [PATCH 2/2] [SCEV] BECount to zero if `((-1 + (1 smax %x))<nsw> /u
%x)` holds
The SCEV expression `((-1 + (1 smax %x))<nsw> /u %x)` can be folded
to zero for positive %x, and when the add node is guaranteed not to
wrap in signed sense.
---
llvm/lib/Analysis/ScalarEvolution.cpp | 19 +++++++++++++++++++
.../udiv-of-x-xsmaxone-fold.ll | 4 ++--
2 files changed, 21 insertions(+), 2 deletions(-)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 487844f000ac69..d046fb4f2e7a99 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -3547,6 +3547,25 @@ const SCEV *ScalarEvolution::getUDivExpr(const SCEV *LHS,
}
}
+ // ((-1 + (1 smax %x))<nsw> /u %x) evaluates to zero, for any positive %x.
+ if (const auto *AE = dyn_cast<SCEVAddExpr>(LHS);
+ AE && AE->getNumOperands() == 2 && AE->hasNoSignedWrap()) {
+ bool FoundMinusOne = false, FoundOneSMaxRHS = false;
+ for (const SCEV *Op : AE->operands()) {
+ if (Op->isAllOnesValue()) {
+ FoundMinusOne = true;
+ } else if (const auto *MME = dyn_cast<SCEVMinMaxExpr>(Op)) {
+ if (MME->getNumOperands() == 2 && MME->getSCEVType() == scSMaxExpr)
+ if (hasOperand(Op, RHS) && hasOperand(Op, getOne(RHS->getType())))
+ FoundOneSMaxRHS = true;
+ }
+ }
+
+ if (FoundMinusOne && FoundOneSMaxRHS)
+ // The backedge is never taken.
+ return getZero(LHS->getType());
+ }
+
// The Insertion Point (IP) might be invalid by now (due to UniqueSCEVs
// changes). Make sure we get a new one.
IP = nullptr;
diff --git a/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll b/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll
index a9f6c1e62dfbc3..a884a66dcf5d3c 100644
--- a/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll
+++ b/llvm/test/Analysis/ScalarEvolution/udiv-of-x-xsmaxone-fold.ll
@@ -13,9 +13,9 @@ define void @test(i32 noundef %x) {
; CHECK-NEXT: %0 = add nsw i32 %smax, -1
; CHECK-NEXT: --> (-1 + (1 smax %x))<nsw> U: [0,2147483647) S: [0,2147483647)
; CHECK-NEXT: %1 = udiv i32 %0, %x
-; CHECK-NEXT: --> ((-1 + (1 smax %x))<nsw> /u %x) U: [0,2147483647) S: [0,2147483647)
+; CHECK-NEXT: --> 0 U: [0,1) S: [0,1)
; CHECK-NEXT: %2 = add nuw nsw i32 %1, 1
-; CHECK-NEXT: --> (1 + ((-1 + (1 smax %x))<nsw> /u %x))<nuw><nsw> U: [1,-2147483648) S: [1,-2147483648)
+; CHECK-NEXT: --> 1 U: [1,2) S: [1,2)
;
; CHECK: Determining loop execution counts for: @test
; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
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