Reapply r237539 with a fix for the Chromium build.

Make sure if we're truncating a constant that would then be sign extended
that the sign extension of the truncated constant is the same as the
original constant.

> Canonicalize min/max expressions correctly.
>
> This patch introduces a canonical form for min/max idioms where one operand
> is extended or truncated. This often happens when the other operand is a
> constant. For example:
>
> %1 = icmp slt i32 %a, i32 0
> %2 = sext i32 %a to i64
> %3 = select i1 %1, i64 %2, i64 0
>
> Would now be canonicalized into:
>
> %1 = icmp slt i32 %a, i32 0
> %2 = select i1 %1, i32 %a, i32 0
> %3 = sext i32 %2 to i64
>
> This builds upon a patch posted by David Majenemer
> (https://www.marc.info/?l=llvm-commits&m=143008038714141&w=2). That pass
> passively stopped instcombine from ruining canonical patterns. This
> patch additionally actively makes instcombine canonicalize too.
>
> Canonicalization of expressions involving a change in type from int->fp
> or fp->int are not yet implemented.

llvm-svn: 237821

1 files changed, 9 insertions, 2 deletions

diff --git a/llvm/lib/Analysis/ValueTracking.cpp b/llvm/lib/Analysis/ValueTracking.cpp
index d0660f6d965..a55712c6296 100644
--- a/llvm/lib/Analysis/ValueTracking.cpp
+++ b/llvm/lib/Analysis/ValueTracking.cpp
@@ -3394,8 +3394,15 @@ static Constant *lookThroughCast(ICmpInst *CmpI, Value *V1, Value *V2,
     return nullptr;
   *CastOp = CI->getOpcode();
 
-  if ((isa<SExtInst>(CI) && CmpI->isSigned()) ||
-      (isa<ZExtInst>(CI) && CmpI->isUnsigned()))
+  if (isa<SExtInst>(CI) && CmpI->isSigned()) {
+    Constant *T = ConstantExpr::getTrunc(C, CI->getSrcTy());
+    // This is only valid if the truncated value can be sign-extended
+    // back to the original value.
+    if (ConstantExpr::getSExt(T, C->getType()) == C)
+      return T;
+    return nullptr;
+  }
+  if (isa<ZExtInst>(CI) && CmpI->isUnsigned())
     return ConstantExpr::getTrunc(C, CI->getSrcTy());
 
   if (isa<TruncInst>(CI))