[llvm] [BasicAA] Treat returns_twice functions as clobbering unescaped objects (PR #117902)
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Thu Nov 28 01:04:39 PST 2024
https://github.com/nikic updated https://github.com/llvm/llvm-project/pull/117902
>From 6ff890f56aec358097a441625b62db8b52d2a01b Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Wed, 27 Nov 2024 17:22:08 +0100
Subject: [PATCH 1/2] [BasicAA] Treat returns_twice functions as clobbering
unescaped objects
Effectively this models all the accesses that occur between the
first and second return as happening at the point of the call.
I left this a generic check, though at least for setjmp and C
semantics specifically, we could skip allocas, as these are
required to use volatile accesses when modified between setjmp
and longjmp.
---
llvm/lib/Analysis/BasicAliasAnalysis.cpp | 6 +++++-
llvm/test/Transforms/GVN/setjmp.ll | 4 ++--
2 files changed, 7 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index 381fb7bbdb5171..e5f71753005f42 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -947,8 +947,12 @@ ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
//
// Make sure the object has not escaped here, and then check that none of the
// call arguments alias the object below.
+ //
+ // We model calls that can return twice (setjmp) as clobbering non-escaping
+ // objects, to model any accesses that may occur prior to the second return.
if (!isa<Constant>(Object) && Call != Object &&
- AAQI.CA->isNotCapturedBefore(Object, Call, /*OrAt*/ false)) {
+ AAQI.CA->isNotCapturedBefore(Object, Call, /*OrAt*/ false) &&
+ !Call->hasFnAttr(Attribute::ReturnsTwice)) {
// Optimistically assume that call doesn't touch Object and check this
// assumption in the following loop.
diff --git a/llvm/test/Transforms/GVN/setjmp.ll b/llvm/test/Transforms/GVN/setjmp.ll
index 0277fcfa226ed6..0ebe24879d320c 100644
--- a/llvm/test/Transforms/GVN/setjmp.ll
+++ b/llvm/test/Transforms/GVN/setjmp.ll
@@ -5,7 +5,6 @@ declare i32 @setjmp() returns_twice
declare void @longjmp()
declare ptr @malloc(i64)
-; FIXME: This is a miscompile.
define i32 @test() {
; CHECK-LABEL: define i32 @test() {
; CHECK-NEXT: [[MALLOC:%.*]] = call noalias ptr @malloc(i64 4)
@@ -18,7 +17,8 @@ define i32 @test() {
; CHECK-NEXT: call void @longjmp()
; CHECK-NEXT: unreachable
; CHECK: [[IF_END]]:
-; CHECK-NEXT: ret i32 10
+; CHECK-NEXT: [[RES:%.*]] = load i32, ptr [[MALLOC]], align 4
+; CHECK-NEXT: ret i32 [[RES]]
;
%malloc = call noalias ptr @malloc(i64 4)
store i32 10, ptr %malloc, align 4
>From a91679408b1d6de9575c3daf76d735584ad1e1f1 Mon Sep 17 00:00:00 2001
From: Nikita Popov <npopov at redhat.com>
Date: Thu, 28 Nov 2024 10:04:18 +0100
Subject: [PATCH 2/2] add alloca exception
---
llvm/lib/Analysis/BasicAliasAnalysis.cpp | 4 +-
llvm/test/Transforms/GVN/setjmp.ll | 62 ++++++++++++++++++++++++
2 files changed, 65 insertions(+), 1 deletion(-)
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index e5f71753005f42..648a22deaf6ba4 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -950,9 +950,11 @@ ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
//
// We model calls that can return twice (setjmp) as clobbering non-escaping
// objects, to model any accesses that may occur prior to the second return.
+ // As an exception, ignore allocas, as setjmp is not required to preserve
+ // non-volatile stores for them.
if (!isa<Constant>(Object) && Call != Object &&
AAQI.CA->isNotCapturedBefore(Object, Call, /*OrAt*/ false) &&
- !Call->hasFnAttr(Attribute::ReturnsTwice)) {
+ (isa<AllocaInst>(Object) || !Call->hasFnAttr(Attribute::ReturnsTwice))) {
// Optimistically assume that call doesn't touch Object and check this
// assumption in the following loop.
diff --git a/llvm/test/Transforms/GVN/setjmp.ll b/llvm/test/Transforms/GVN/setjmp.ll
index 0ebe24879d320c..07b7028346760b 100644
--- a/llvm/test/Transforms/GVN/setjmp.ll
+++ b/llvm/test/Transforms/GVN/setjmp.ll
@@ -35,3 +35,65 @@ if.end:
%res = load i32, ptr %malloc
ret i32 %res
}
+
+; We are still allowed to optimize non-volatile accesses to allocas.
+define i32 @test_alloca() {
+; CHECK-LABEL: define i32 @test_alloca() {
+; CHECK-NEXT: [[ALLOC:%.*]] = alloca i43, align 8
+; CHECK-NEXT: store i32 10, ptr [[ALLOC]], align 4
+; CHECK-NEXT: [[SJ:%.*]] = call i32 @setjmp()
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[SJ]], 0
+; CHECK-NEXT: br i1 [[CMP]], label %[[IF_THEN:.*]], label %[[IF_END:.*]]
+; CHECK: [[IF_THEN]]:
+; CHECK-NEXT: store i32 20, ptr [[ALLOC]], align 4
+; CHECK-NEXT: call void @longjmp()
+; CHECK-NEXT: unreachable
+; CHECK: [[IF_END]]:
+; CHECK-NEXT: ret i32 10
+;
+ %alloc = alloca i43
+ store i32 10, ptr %alloc, align 4
+ %sj = call i32 @setjmp()
+ %cmp = icmp eq i32 %sj, 0
+ br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+ store i32 20, ptr %alloc
+ call void @longjmp()
+ unreachable
+
+if.end:
+ %res = load i32, ptr %alloc
+ ret i32 %res
+}
+
+define i32 @test_alloca_volatile() {
+; CHECK-LABEL: define i32 @test_alloca_volatile() {
+; CHECK-NEXT: [[ALLOC:%.*]] = alloca i43, align 8
+; CHECK-NEXT: store volatile i32 10, ptr [[ALLOC]], align 4
+; CHECK-NEXT: [[SJ:%.*]] = call i32 @setjmp()
+; CHECK-NEXT: [[CMP:%.*]] = icmp eq i32 [[SJ]], 0
+; CHECK-NEXT: br i1 [[CMP]], label %[[IF_THEN:.*]], label %[[IF_END:.*]]
+; CHECK: [[IF_THEN]]:
+; CHECK-NEXT: store volatile i32 20, ptr [[ALLOC]], align 4
+; CHECK-NEXT: call void @longjmp()
+; CHECK-NEXT: unreachable
+; CHECK: [[IF_END]]:
+; CHECK-NEXT: [[RES:%.*]] = load volatile i32, ptr [[ALLOC]], align 4
+; CHECK-NEXT: ret i32 [[RES]]
+;
+ %alloc = alloca i43
+ store volatile i32 10, ptr %alloc, align 4
+ %sj = call i32 @setjmp()
+ %cmp = icmp eq i32 %sj, 0
+ br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+ store volatile i32 20, ptr %alloc
+ call void @longjmp()
+ unreachable
+
+if.end:
+ %res = load volatile i32, ptr %alloc
+ ret i32 %res
+}
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