[PATCH] D24104: Make GlobalsAA ignore dead constant expressions.

[Eli Friedman via llvm-commits]

efriedma created this revision.
efriedma added reviewers: jmolloy, sanjoy, mkuper.
efriedma added a subscriber: llvm-commits.
efriedma set the repository for this revision to rL LLVM.

Slightly improves the precision of GlobalsAA in certain situations, and makes the behavior of optimization passes more predictable.

Repository:
  rL LLVM

https://reviews.llvm.org/D24104

Files:
  lib/Analysis/GlobalsModRef.cpp
  test/Analysis/GlobalsModRef/dead-uses.ll

Index: test/Analysis/GlobalsModRef/dead-uses.ll
===================================================================
--- /dev/null
+++ test/Analysis/GlobalsModRef/dead-uses.ll
@@ -0,0 +1,52 @@
+; RUN: opt < %s -instcombine -globals-aa -licm -S | FileCheck %s
+
+; Make sure -globals-aa ignores dead uses of globals.
+
+ at a = internal global i32 0, align 4
+ at c = common global i32 0, align 4
+
+; Function Attrs: nounwind
+define i32 @g() #0 {
+; CHECK-LABEL: define i32 @g()
+; CHECK: load i32, i32* @a, align 4
+; CHECK-NEXT: br label %for.cond
+entry:
+  br label %for.cond
+
+for.cond:                                         ; preds = %for.inc, %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %for.inc ]
+  %sum.0 = phi i32 [ 0, %entry ], [ %add, %for.inc ]
+  %cmp = icmp slt i32 %i.0, 1000
+  br i1 %cmp, label %for.body, label %for.end
+
+for.body:                                         ; preds = %for.cond
+  %0 = load i32, i32* @a, align 4
+  %add = add nsw i32 %sum.0, %0
+  call void @f()
+  br label %for.inc
+
+for.inc:                                          ; preds = %for.body
+  %inc = add nsw i32 %i.0, 1
+  br label %for.cond
+
+for.end:                                          ; preds = %for.cond
+  ret i32 %sum.0
+}
+
+; Function Attrs: nounwind
+define internal void @f() #0 {
+entry:
+  %tobool = icmp ne i32 0, 0
+  br i1 %tobool, label %if.then, label %if.end
+
+if.then:                                          ; preds = %entry
+  store i32 ptrtoint (i32* @a to i32), i32* @c, align 4
+  br label %if.end
+
+if.end:                                           ; preds = %if.then, %entry
+  %0 = load i32, i32* @c, align 4
+  %inc = add nsw i32 %0, 1
+  store i32 %inc, i32* @c, align 4
+  ret void
+}
+
Index: lib/Analysis/GlobalsModRef.cpp
===================================================================
--- lib/Analysis/GlobalsModRef.cpp
+++ lib/Analysis/GlobalsModRef.cpp
@@ -272,19 +272,24 @@
 void GlobalsAAResult::AnalyzeGlobals(Module &M) {
   SmallPtrSet<Function *, 32> TrackedFunctions;
   for (Function &F : M)
-    if (F.hasLocalLinkage())
+    if (F.hasLocalLinkage()) {
+      // AnalyzeUsesOfPointer assumes all uses of the value are live.
+      F.removeDeadConstantUsers();
       if (!AnalyzeUsesOfPointer(&F)) {
         // Remember that we are tracking this global.
         NonAddressTakenGlobals.insert(&F);
         TrackedFunctions.insert(&F);
         Handles.emplace_front(*this, &F);
         Handles.front().I = Handles.begin();
         ++NumNonAddrTakenFunctions;
       }
+    }
 
   SmallPtrSet<Function *, 16> Readers, Writers;
   for (GlobalVariable &GV : M.globals())
     if (GV.hasLocalLinkage()) {
+      // AnalyzeUsesOfPointer assumes all uses of the value are live.
+      GV.removeDeadConstantUsers();
       if (!AnalyzeUsesOfPointer(&GV, &Readers,
                                 GV.isConstant() ? nullptr : &Writers)) {
         // Remember that we are tracking this global, and the mod/ref fns


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