[llvm] [CGP] Undo constant propagation of pointers across calls (PR #102926)
Antonio Frighetto via llvm-commits
llvm-commits at lists.llvm.org
Mon Aug 26 06:49:03 PDT 2024
https://github.com/antoniofrighetto updated https://github.com/llvm/llvm-project/pull/102926
>From 08c21613376c2e7dd806545bb9d5af630ce082f4 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Mon, 26 Aug 2024 10:00:45 +0200
Subject: [PATCH 1/3] [CGP] Introduce test for PR102926 (NFC)
---
...evert-constant-ptr-propagation-on-calls.ll | 54 +++++++++++++++++++
1 file changed, 54 insertions(+)
create mode 100644 llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll
diff --git a/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll b/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll
new file mode 100644
index 00000000000000..d1311c46be8a0c
--- /dev/null
+++ b/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes='require<profile-summary>,function(codegenprepare)' -mtriple=aarch64-none-linux-gnu < %s | FileCheck %s
+
+%struct.S = type { i8 }
+
+ at g_getS = internal global %struct.S zeroinitializer, align 1
+ at guard = internal global i64 0, align 8
+
+define nonnull align 1 dereferenceable(1) ptr @getS() personality ptr @__gxx_personality_v0 {
+entry:
+ %guard = load atomic i8, ptr @guard acquire, align 8
+ %mask = and i8 %guard, 1
+ %cond = icmp eq i8 %mask, 0
+ br i1 %cond, label %to_be_init, label %return
+
+to_be_init: ; preds = %entry
+ %is_init = call i32 @__cxa_guard_acquire(ptr @guard)
+ %cond.2 = icmp ne i32 %is_init, 0
+ br i1 %cond.2, label %ctor, label %return
+
+ctor: ; preds = %to_be_init
+ invoke void @S_ctor(ptr nonnull align 1 dereferenceable(1) @g_getS)
+ to label %continue unwind label %landing_pad
+
+continue: ; preds = %ctor
+ call void @__cxa_guard_release(ptr @guard)
+ br label %return
+
+return: ; preds = %continue, %to_be_init, %entry
+ ret ptr @g_getS
+
+landing_pad: ; preds = %ctor
+ %lp = landingpad { ptr, i32 } cleanup
+ call void @__cxa_guard_abort(ptr @guard)
+ resume { ptr, i32 } %lp
+}
+
+define i32 @getI() {
+; CHECK-LABEL: @getI(
+; CHECK-NEXT: [[GETS_PTR:%.*]] = call nonnull align 1 dereferenceable(1) ptr @getS()
+; CHECK-NEXT: [[GETI:%.*]] = call i32 @S_getI(ptr nonnull align 1 dereferenceable(1) @g_getS)
+; CHECK-NEXT: ret i32 [[GETI]]
+;
+ %getS_ptr = call nonnull align 1 dereferenceable(1) ptr @getS()
+ %getI = call i32 @S_getI(ptr nonnull align 1 dereferenceable(1) @g_getS)
+ ret i32 %getI
+}
+
+declare i32 @__cxa_guard_acquire(ptr)
+declare void @S_ctor(ptr nonnull align 1 dereferenceable(1))
+declare i32 @S_getI(ptr nonnull align 1 dereferenceable(1))
+declare void @__cxa_guard_abort(ptr)
+declare void @__cxa_guard_release(ptr)
+declare i32 @__gxx_personality_v0(...)
>From fca0a4d84c6fc0965e2a6f7dbabb425101dd2f59 Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Sun, 25 Aug 2024 16:24:19 +0200
Subject: [PATCH 2/3] [CGP] Undo constant propagation of pointers across calls
It may be profitable to revert SCCP propagation of C++ static values,
if such constants are pointers, in order to avoid redundant pointer
computation, since the method returning the constant is non-removable.
---
llvm/lib/CodeGen/CodeGenPrepare.cpp | 41 +++++++++++++++++++
...evert-constant-ptr-propagation-on-calls.ll | 2 +-
2 files changed, 42 insertions(+), 1 deletion(-)
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index bf48c1fdab0ff0..65956ba54b8998 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -2686,6 +2686,47 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT) {
return true;
}
+ // SCCP may have propagated C++ static variables across calls. If this happens
+ // to be the case, we may want to undo it in order to avoid redundant pointer
+ // computation of the constant, as the function method returning the constant
+ // needs to be executed anyways.
+ auto GetUniformReturnValue = [](const Function *F) -> Constant * {
+ if (!F->getReturnType()->isPointerTy())
+ return nullptr;
+
+ Constant *UniformValue = nullptr;
+ for (auto &BB : llvm::reverse(*F)) {
+ if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
+ if (auto *V = dyn_cast_or_null<GlobalVariable>(RI->getReturnValue())) {
+ if (!UniformValue)
+ UniformValue = V;
+ else if (V != UniformValue)
+ return nullptr;
+ } else {
+ return nullptr;
+ }
+ }
+ }
+
+ return UniformValue;
+ };
+
+ if (Constant *RV = GetUniformReturnValue(CI->getCalledFunction())) {
+ bool MadeChange = false;
+ const auto &DT = getDT(*CI->getFunction());
+ for (Use &U : make_early_inc_range(RV->uses())) {
+ auto *I = dyn_cast<Instruction>(U.getUser());
+ if (!I || I->getParent() != CI->getParent())
+ continue;
+ if (DT.dominates(CI, I)) {
+ U.set(CI);
+ MadeChange = true;
+ }
+ }
+
+ return MadeChange;
+ }
+
return false;
}
diff --git a/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll b/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll
index d1311c46be8a0c..e38a22a30df056 100644
--- a/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll
+++ b/llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll
@@ -38,7 +38,7 @@ landing_pad: ; preds = %ctor
define i32 @getI() {
; CHECK-LABEL: @getI(
; CHECK-NEXT: [[GETS_PTR:%.*]] = call nonnull align 1 dereferenceable(1) ptr @getS()
-; CHECK-NEXT: [[GETI:%.*]] = call i32 @S_getI(ptr nonnull align 1 dereferenceable(1) @g_getS)
+; CHECK-NEXT: [[GETI:%.*]] = call i32 @S_getI(ptr nonnull align 1 dereferenceable(1) [[GETS_PTR]])
; CHECK-NEXT: ret i32 [[GETI]]
;
%getS_ptr = call nonnull align 1 dereferenceable(1) ptr @getS()
>From ed5ba40889d515f82e231e3280da34f65b84641c Mon Sep 17 00:00:00 2001
From: Antonio Frighetto <me at antoniofrighetto.com>
Date: Mon, 26 Aug 2024 15:48:35 +0200
Subject: [PATCH 3/3] !fixup drop no longer needed check
---
llvm/lib/CodeGen/CodeGenPrepare.cpp | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 65956ba54b8998..4705c0f8a78fe0 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -2697,7 +2697,7 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT) {
Constant *UniformValue = nullptr;
for (auto &BB : llvm::reverse(*F)) {
if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
- if (auto *V = dyn_cast_or_null<GlobalVariable>(RI->getReturnValue())) {
+ if (auto *V = dyn_cast<GlobalVariable>(RI->getReturnValue())) {
if (!UniformValue)
UniformValue = V;
else if (V != UniformValue)
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