[SCCP] Force unknown branches to overdefined

[davide]

Summary

See https://llvm.org/bugs/show_bug.cgi?id=30448 for the full discussion, and thanks to Eli helping finding the problem

[davide]

This fixes the miscompile reported in PR30448. I tried on many internal tests and bootstrapping clang, and it works without problems. We lose some optimization power, but it seems that the pattern is rare enough so we shouldn't really worry.

lib/Transforms/Scalar/SCCP.cpp

       if (isa<UndefValue>(BI->getCondition())) {
         BI->setCondition(ConstantInt::getFalse(BI->getContext()));
         markEdgeExecutable(&BB, TI->getSuccessor(1));
         return true;
       }
 
       // Otherwise, it is a branch on a symbolic value which is currently
       // considered to be undef.  Handle this by forcing the input value to the
-      // branch to false.
-      markForcedConstant(BI->getCondition(),
-                         ConstantInt::getFalse(TI->getContext()));
+      // branch to overdefined.
+      markOverdefined(BI->getCondition());
       return true;
     }
 
     if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
       if (!SI->getNumCases() || !getValueState(SI->getCondition()).isUnknown())
         continue;
 
       // If the input to SCCP is actually switch on undef, fix the undef to

test/Transforms/SCCP/apint-ipsccp3.ll

-; RUN: opt < %s -ipsccp -S | not grep global
-
-@G = internal global i66 undef
+; RUN: opt < %s -ipsccp -S | FileCheck %s
 
+; CHECK: @G = internal global i66 undef
+; CHECK: define void @foo() {
+; CHECK:   %X = load i66, i66* @G
+; CHECK:   store i66 %X, i66* @G
+; CHECK:   ret void
+; CHECK: }
+; CHECK: define i66 @bar() {
+; CHECK:   %V = load i66, i66* @G
+; CHECK:   %C = icmp eq i66 %V, 17
+; CHECK:   br i1 %C, label %T, label %F
+; CHECK: T:
+; CHECK:   store i66 17, i66* @G
+; CHECK:   ret i66 %V
+; CHECK: F:
+; CHECK:   store i66 123, i66* @G
+; CHECK:   ret i66 0
+; CHECK: }
 
+@G = internal global i66 undef
 
 define void @foo() {
   %X = load i66, i66* @G
   store i66 %X, i66* @G
   ret void
 }
 
 define i66 @bar() {

test/Transforms/SCCP/ipsccp-basic.ll

 @G = internal global i32 undef
 
 define void @test3a() {
   %X = load i32, i32* @G
   store i32 %X, i32* @G
   ret void
 }
 ; CHECK-LABEL: define void @test3a(
-; CHECK-NEXT: ret void
+; CHECK-NEXT:   %X = load i32, i32* @G
+; CHECK-NEXT:   store i32 %X, i32* @G
+; CHECK-NEXT:   ret void
 
-
 define i32 @test3b() {
   %V = load i32, i32* @G
   %C = icmp eq i32 %V, 17
   br i1 %C, label %T, label %F
 T:
   store i32 17, i32* @G
   ret i32 %V
 F:  
   store i32 123, i32* @G
   ret i32 0
 }
 ; CHECK-LABEL: define i32 @test3b(
-; CHECK-NOT: store
+; CHECK: store i32 17, i32* @G
+; CHECK: store i32 123, i32* @G
 ; CHECK: ret i32 0
 
 
 ;;======================== test4
 
 define internal {i64,i64} @test4a() {
   %a = insertvalue {i64,i64} undef, i64 4, 1
   %b = insertvalue {i64,i64} %a, i64 5, 0