[llvm] [GlobalOpt] Remove all stores to GV (PR #84694)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Anshil Gandhi (gandhi56)

<details>
<summary>Changes</summary>

When CleanupPointerRootUsers is called on GV,
the assumption is that there is no load from GV.
Erase all stores to GV in this case.

Fixes https://github.com/llvm/llvm-project/issues/64680.

---
Full diff: https://github.com/llvm/llvm-project/pull/84694.diff


3 Files Affected:

- (modified) llvm/lib/Transforms/IPO/GlobalOpt.cpp (+9-1) 
- (added) llvm/test/Transforms/GlobalOpt/cleanup-pointer-root-stores.ll (+62) 
- (modified) llvm/test/Transforms/GlobalOpt/dead-store-status.ll (-2) 


``````````diff
diff --git a/llvm/lib/Transforms/IPO/GlobalOpt.cpp b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
index da714c9a75701b..2b3154eeb6100f 100644
--- a/llvm/lib/Transforms/IPO/GlobalOpt.cpp
+++ b/llvm/lib/Transforms/IPO/GlobalOpt.cpp
@@ -204,8 +204,9 @@ CleanupPointerRootUsers(GlobalVariable *GV,
   // If Dead[n].first is the only use of a malloc result, we can delete its
   // chain of computation and the store to the global in Dead[n].second.
   SmallVector<std::pair<Instruction *, Instruction *>, 32> Dead;
-
+  SmallVector<StoreInst *> StoresToGV;
   SmallVector<User *> Worklist(GV->users());
+
   // Constants can't be pointers to dynamically allocated memory.
   while (!Worklist.empty()) {
     User *U = Worklist.pop_back_val();
@@ -217,6 +218,8 @@ CleanupPointerRootUsers(GlobalVariable *GV,
       } else if (Instruction *I = dyn_cast<Instruction>(V)) {
         if (I->hasOneUse())
           Dead.push_back(std::make_pair(I, SI));
+        else
+          StoresToGV.push_back(SI);
       }
     } else if (MemSetInst *MSI = dyn_cast<MemSetInst>(U)) {
       if (isa<Constant>(MSI->getValue())) {
@@ -241,6 +244,11 @@ CleanupPointerRootUsers(GlobalVariable *GV,
     }
   }
 
+  // We assert here that GV is never load from so
+  // we can safely remove all stores to GV.
+  for (StoreInst *SI : StoresToGV)
+    SI->eraseFromParent();
+
   for (int i = 0, e = Dead.size(); i != e; ++i) {
     if (IsSafeComputationToRemove(Dead[i].first, GetTLI)) {
       Dead[i].second->eraseFromParent();
diff --git a/llvm/test/Transforms/GlobalOpt/cleanup-pointer-root-stores.ll b/llvm/test/Transforms/GlobalOpt/cleanup-pointer-root-stores.ll
new file mode 100644
index 00000000000000..980fb6f5dc8bfe
--- /dev/null
+++ b/llvm/test/Transforms/GlobalOpt/cleanup-pointer-root-stores.ll
@@ -0,0 +1,62 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -passes=globalopt < %s -S | FileCheck %s
+
+ at a = internal unnamed_addr global i32 0, align 4
+ at b = internal unnamed_addr global [3 x ptr] zeroinitializer, align 16
+
+define i32 @main() local_unnamed_addr {
+; CHECK-LABEL: define i32 @main() local_unnamed_addr {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[E:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    [[DOTPR:%.*]] = load i32, ptr @a, align 4
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp slt i32 [[DOTPR]], 3
+; CHECK-NEXT:    br i1 [[CMP1]], label [[FOR_BODY:%.*]], label [[FOR_END:%.*]]
+; CHECK:       for.body:
+; CHECK-NEXT:    store i32 8, ptr [[E]], align 4
+; CHECK-NEXT:    call void @bar20_()
+; CHECK-NEXT:    [[TMP0:%.*]] = load i32, ptr [[E]], align 4
+; CHECK-NEXT:    [[TOBOOL_NOT:%.*]] = icmp eq i32 [[TMP0]], 0
+; CHECK-NEXT:    br i1 [[TOBOOL_NOT]], label [[IF_THEN:%.*]], label [[IF_END:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[IF_END]]
+; CHECK:       if.end:
+; CHECK-NEXT:    [[TMP1:%.*]] = load i32, ptr @a, align 4
+; CHECK-NEXT:    [[INC:%.*]] = add nsw i32 [[TMP1]], 1
+; CHECK-NEXT:    store i32 [[INC]], ptr @a, align 4
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[TMP1]], 2
+; CHECK-NEXT:    br i1 [[CMP]], label [[FOR_BODY]], label [[FOR_END]]
+; CHECK:       for.end:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %e = alloca i32, align 4
+  %.pr = load i32, ptr @a, align 4
+  %cmp1 = icmp slt i32 %.pr, 3
+  br i1 %cmp1, label %for.body, label %for.end
+
+for.body:                                         ; preds = %entry, %if.end
+  store i32 8, ptr %e, align 4
+  call void @bar20_()
+  %0 = load i32, ptr %e, align 4
+  %tobool.not = icmp eq i32 %0, 0
+  br i1 %tobool.not, label %if.then, label %if.end
+
+if.then:                                          ; preds = %for.body
+  call void @foo()
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %for.body
+  store ptr %e, ptr getelementptr inbounds ([3 x ptr], ptr @b, i64 0, i64 2), align 16
+  %1 = load i32, ptr @a, align 4
+  %inc = add nsw i32 %1, 1
+  store i32 %inc, ptr @a, align 4
+  %cmp = icmp slt i32 %1, 2
+  br i1 %cmp, label %for.body, label %for.end
+
+for.end:                                          ; preds = %if.end, %entry
+  ret i32 0
+}
+
+declare void @bar20_() local_unnamed_addr
+declare void @foo() local_unnamed_addr
diff --git a/llvm/test/Transforms/GlobalOpt/dead-store-status.ll b/llvm/test/Transforms/GlobalOpt/dead-store-status.ll
index 9a8fbb8d65f0e0..597c08929af902 100644
--- a/llvm/test/Transforms/GlobalOpt/dead-store-status.ll
+++ b/llvm/test/Transforms/GlobalOpt/dead-store-status.ll
@@ -4,8 +4,6 @@
 ; false. This was caught by the pass return status check that is hidden under
 ; EXPENSIVE_CHECKS.
 
-; CHECK: @global = internal unnamed_addr global ptr null, align 1
-
 ; CHECK-LABEL: @foo
 ; CHECK-NEXT: entry:
 ; CHECK-NEXT: ret i16 undef

``````````

</details>


https://github.com/llvm/llvm-project/pull/84694