[SCCP] Do not mark unknown loads as overdefined.

For tracked globals that are unknown after solving, we expect all
non-store uses to be replaced.

This is a follow-up to f8045b250d, which removed forcedconstant.

We should not mark unknown loads as overdefined, as they either load
from an unknown pointer or an undef global. Restore the original logic
for loads.

llvm/lib/Transforms/Scalar/SCCP.cpp

@@ -1501,6 +1501,13 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
           if (TrackedRetVals.count(F))
             continue;
 
+      if (isa<LoadInst>(I)) {
+        // A load here means one of two things: a load of undef from a global,
+        // a load from an unknown pointer.  Either way, having it return undef
+        // is okay.
+        continue;
+      }
+
       markOverdefined(&I);
       return true;
     }

llvm/test/Transforms/IPConstantProp/PR26044.ll

@@ -11,8 +11,7 @@ define void @fn2(i32* %P) {
 ; CHECK:       for.cond1:
 ; CHECK-NEXT:    br i1 false, label [[IF_END]], label [[IF_END]]
 ; CHECK:       if.end:
-; CHECK-NEXT:    [[TMP0:%.*]] = load i32, i32* null, align 4
-; CHECK-NEXT:    [[CALL:%.*]] = call i32 @fn1(i32 [[TMP0]])
+; CHECK-NEXT:    [[CALL:%.*]] = call i32 @fn1(i32 undef)
 ; CHECK-NEXT:    store i32 [[CALL]], i32* [[P]]
 ; CHECK-NEXT:    br label [[FOR_COND1:%.*]]
 ;
@@ -34,8 +33,8 @@ define internal i32 @fn1(i32 %p1) {
 ; CHECK-LABEL: define {{[^@]+}}@fn1
 ; CHECK-SAME: (i32 [[P1:%.*]])
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i32 [[P1]], 0
-; CHECK-NEXT:    [[COND:%.*]] = select i1 [[TOBOOL]], i32 [[P1]], i32 [[P1]]
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp ne i32 undef, 0
+; CHECK-NEXT:    [[COND:%.*]] = select i1 [[TOBOOL]], i32 undef, i32 undef
 ; CHECK-NEXT:    ret i32 [[COND]]
 ;
 entry:

llvm/test/Transforms/SCCP/apint-bigint2.ll

@@ -18,8 +18,7 @@ define i101 @array() {
 }
 
 ; CHECK-LABEL: @large_aggregate
-; CHECK-NEXT:    %B = load i101, i101* undef
-; CHECK-NEXT:    %D = and i101 %B, 1
+; CHECK-NEXT:    %D = and i101 undef, 1
 ; CHECK-NEXT:    %DD = or i101 %D, 1
 ; CHECK-NEXT:    %G = getelementptr i101, i101* getelementptr inbounds ([6 x i101], [6 x i101]* @Y, i32 0, i32 5), i101 %DD
 ; CHECK-NEXT:    %L3 = load i101, i101* %G

llvm/test/Transforms/SCCP/loadtest.ll

@@ -2,6 +2,7 @@
 
 ; RUN: opt < %s -data-layout="e-p:32:32" -debugify -sccp -S | FileCheck %s
 ; RUN: opt < %s -data-layout="E-p:32:32" -debugify -sccp -S | FileCheck %s
+; RUN: opt < %s -data-layout="E-p:32:32" -debugify -ipsccp -S | FileCheck %s
 
 @X = constant i32 42		; <i32*> [#uses=1]
 @Y = constant [2 x { i32, float }] [ { i32, float } { i32 12, float 1.000000e+00 }, { i32, float } { i32 37, float 0x3FF3B2FEC0000000 } ]		; <[2 x { i32, float }]*> [#uses=2]
@@ -43,4 +44,3 @@ define i8 @test4() {
 	%B = load i8, i8* %A
 	ret i8 %B
 }
-

llvm/test/Transforms/SCCP/loadtest2.ll

@@ -0,0 +1,14 @@
+; RUN: opt < %s -data-layout="E-p:32:32" -ipsccp -S | FileCheck %s
+
+@j = internal global i32 undef, align 4
+
+; Make sure we do not mark loads from undef as overdefined.
+define i32 @test5(i32 %b) {
+; CHECK-LABEL: define i32 @test5(i32 %b)
+; CHECK-NEXT:    %add = add nsw i32 undef, %b
+; CHECK-NEXT:    ret i32 %add
+;
+  %l = load i32, i32* @j, align 4
+  %add = add nsw i32 %l, %b
+  ret i32 %add
+}