[Attributor] Fix for a crash on RAUW when rewriting function signature

[uenoku]

Looks reasonable to me.

[jdoerfert]

The fix is somewhat counter intuitive or I do not understand it. Checking if we have uses before replacing *all uses* seems redundant. I guess the different type was the problem, question is if we need the RAUW at all and if so, if the type might be different in other situations.

llvm/lib/Transforms/IPO/Attributor.cpp

for (auto &It : ArgumentReplacementMap) {
     }
 
     // Eliminate the instructions *after* we visited all of them.
     for (auto &CallSitePair : CallSitePairs) {
       CallBase &OldCB = *CallSitePair.first;
       CallBase &NewCB = *CallSitePair.second;
       ModifiedFns.insert(OldCB.getFunction());
       CGUpdater.replaceCallSite(OldCB, NewCB);
+      if (!OldCB.use_empty())
         OldCB.replaceAllUsesWith(&NewCB);
       OldCB.eraseFromParent();
     }
 
     // Replace the function in the call graph (if any).
     CGUpdater.replaceFunctionWith(*OldFn, *NewFn);
 
     // If the old function was modified and needed to be reanalyzed, the new one
     // does now.

llvm/test/Transforms/Attributor/misc_crash.ll

 ; RUN: opt -attributor -S %s | FileCheck %s
 ; RUN: opt -passes=attributor -S %s | FileCheck %s
 
-@var = internal global [1 x i32] undef
+@var1 = internal global [1 x i32] undef
+@var2 = internal global i32 0
 
 ; CHECK-LABEL: define i32 addrspace(1)* @foo(i32 addrspace(4)* nofree readnone %arg)
 define i32 addrspace(1)* @foo(i32 addrspace(4)* %arg) {
 entry:
   %0 = addrspacecast i32 addrspace(4)* %arg to i32 addrspace(1)*
   ret i32 addrspace(1)* %0
 }
 
 define i32* @func1() {
-  %ptr = call i32* @func1a([1 x i32]* @var)
+  %ptr = call i32* @func1a([1 x i32]* @var1)
   ret i32* %ptr
 }
 
 ; CHECK-LABEL: define internal nonnull align 4 dereferenceable(4) i32* @func1a()
-; CHECK-NEXT: ret i32* getelementptr inbounds ([1 x i32], [1 x i32]* @var, i32 0, i32 0)
+; CHECK-NEXT: ret i32* getelementptr inbounds ([1 x i32], [1 x i32]* @var1, i32 0, i32 0)
 define internal i32* @func1a([1 x i32]* %arg) {
   %ptr = getelementptr inbounds [1 x i32], [1 x i32]* %arg, i64 0, i64 0
   ret i32* %ptr
 }
+
+define internal void @func2a(i32* %0) {
+  store i32 0, i32* %0
+  ret void
+}
+
+; CHECK-LABEL: define i32 @func2()
+; CHECK-NEXT:   tail call void @func2a()
+; CHECK-NEXT:   %1 = load i32, i32* @var2, align 4
+; CHECK-NEXT:   ret i32 %1
+define i32 @func2() {
+  %1 = tail call i32 (i32*, ...) bitcast (void (i32*)* @func2a to i32 (i32*, ...)*)(i32* @var2)
+  %2 = load i32, i32* @var2
+  ret i32 %2
+}