[GlobalOpt] Treat null-check of loaded value as use of global (PR35760)

[vsk]

Summary

PR35760 shows an example program which, when compiled with clang -O0
or gcc at any optimization level, prints '0'. However, llvm transforms
the program in a way that causes it to print '1'.

Fix the issue by having AllUsesOfValueWillTrapIfNull return false when
analyzing a load from a global which is used by an icmp. This special
case was untested [0] so this is just deleting dead code.

An alternative fix might be to change the GlobalStatus analysis for the
global to report "Stored" instead of "StoredOnce". However, "StoredOnce"
is appropriate when only one value other than the initializer is stored
to the global.

[0]
http://lab.llvm.org:8080/coverage/coverage-reports/coverage/Users/buildslave/jenkins/workspace/coverage/llvm-project/llvm/lib/Transforms/IPO/GlobalOpt.cpp.html#L662

https://bugs.llvm.org/show_bug.cgi?id=35760

[efriedma]

LGTM

We could theoretically optimize this to two globals: one for the actual allocation, and a boolean representing whether it's been allocated. But that would be a different transform, and I don't expect you to implement it.

llvm/test/Transforms/GlobalOpt/null-check-is-use-pr35760.ll

47

Please delete unnecessary metadata.

[vsk]

Thanks!

llvm/test/Transforms/GlobalOpt/null-check-is-use-pr35760.ll

47

I'll fix this before committing.

llvm/lib/Transforms/IPO/GlobalOpt.cpp

if (isa<LoadInst>(U)) {
       if (!AllUsesOfValueWillTrapIfNull(CI, PHIs)) return false;
     } else if (const GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(U)) {
       if (!AllUsesOfValueWillTrapIfNull(GEPI, PHIs)) return false;
     } else if (const PHINode *PN = dyn_cast<PHINode>(U)) {
       // If we've already seen this phi node, ignore it, it has already been
       // checked.
       if (PHIs.insert(PN).second && !AllUsesOfValueWillTrapIfNull(PN, PHIs))
         return false;
-    } else if (isa<ICmpInst>(U) &&
-               isa<ConstantPointerNull>(U->getOperand(1))) {
-      // Ignore icmp X, null
     } else {
       //cerr << "NONTRAPPING USE: " << *U;
       return false;
     }
   }
   return true;
 }
 

llvm/test/Transforms/GlobalOpt/null-check-is-use-pr35760.ll

+; RUN: opt -S -globalopt -o - < %s | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@_ZL3g_i = internal global i32* null, align 8
+@.str = private unnamed_addr constant [2 x i8] c"0\00", align 1
+@.str.1 = private unnamed_addr constant [2 x i8] c"1\00", align 1
+
+define dso_local i32 @main() {
+  store i32* null, i32** @_ZL3g_i, align 8
+  call void @_ZL13PutsSomethingv()
+  ret i32 0
+}
+
+; CHECK-LABEL: define {{.*}} @_ZL13PutsSomethingv()
+; CHECK: [[gvLoad:%.*]] = load i32*, i32** @_ZL3g_i
+; CHECK-NEXT: icmp eq i32* [[gvLoad]], null
+define internal void @_ZL13PutsSomethingv() {
+  %1 = load i32*, i32** @_ZL3g_i, align 8
+  %2 = icmp eq i32* %1, null
+  br i1 %2, label %3, label %7
+
+3:                                                ; preds = %0
+  %4 = call noalias i8* @malloc(i64 4) #3
+  %5 = bitcast i8* %4 to i32*
+  store i32* %5, i32** @_ZL3g_i, align 8
+  %6 = call i32 @puts(i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str, i64 0, i64 0))
+  br label %9
+
+7:                                                ; preds = %0
+  %8 = call i32 @puts(i8* getelementptr inbounds ([2 x i8], [2 x i8]* @.str.1, i64 0, i64 0))
+  br label %9
+
+9:                                                ; preds = %7, %3
+  ret void
+}
+
+declare dso_local noalias i8* @malloc(i64)
+
+declare dso_local i32 @puts(i8* nocapture readonly)
+
+!llvm.module.flags = !{!0}
+!llvm.ident = !{!1}
+
+!0 = !{i32 1, !"wchar_size", i32 4}
+!1 = !{!"clang version 11.0.0 (https://github.com/llvm/llvm-project.git 34c0759f846447454714c8faa0e1753a8713637b)"}