Make GlobalsAA ignore dead constant expressions.

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

Differential Revision: https://reviews.llvm.org/D24104

llvm-svn: 283165

2 files changed, 56 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/GlobalsModRef.cpp b/llvm/lib/Analysis/GlobalsModRef.cpp
index 0da888d47d1..d29b9703e9c 100644
--- a/llvm/lib/Analysis/GlobalsModRef.cpp
+++ b/llvm/lib/Analysis/GlobalsModRef.cpp
@@ -366,6 +366,8 @@ bool GlobalsAAResult::AnalyzeUsesOfPointer(Value *V,
     } else if (ICmpInst *ICI = dyn_cast<ICmpInst>(I)) {
       if (!isa<ConstantPointerNull>(ICI->getOperand(1)))
         return true; // Allow comparison against null.
+    } else if (Constant *C = dyn_cast<Constant>(I)) {
+      return C->isConstantUsed();
     } else {
       return true;
     }
diff --git a/llvm/test/Analysis/GlobalsModRef/dead-uses.ll b/llvm/test/Analysis/GlobalsModRef/dead-uses.ll
new file mode 100644
index 00000000000..a96655d48bf
--- /dev/null
+++ b/llvm/test/Analysis/GlobalsModRef/dead-uses.ll
@@ -0,0 +1,54 @@
+; RUN: opt < %s -instcombine -globals-aa -licm -S | FileCheck %s
+
+; Make sure -globals-aa ignores dead uses of globals.
+
+@a = internal global i32 0, align 4
+@c = common global i32 0, align 4
+
+; Function Attrs: nounwind
+define i32 @g() {
+; Make sure the load of @a is hoisted.
+; CHECK-LABEL: define i32 @g()
+; CHECK: entry:
+; CHECK-NEXT: 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() {
+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
+}
+