[PATCH] D22247: [DAGCombine] Make sext(setcc) combine respect getBooleanContents()

[Michael Kuperstein via llvm-commits]

mkuper added a comment.

Thanks, Chandler!


================
Comment at: lib/CodeGen/SelectionDAG/DAGCombiner.cpp:6210-6216
@@ -6206,4 +6209,9 @@
     SDLoc DL(N);
-    SDValue NegOne =
-      DAG.getConstant(APInt::getAllOnesValue(ElementWidth), DL, VT);
+    SDValue TrueVal =
+        (SetCCWidth == 1)
+            ? DAG.getConstant(APInt::getAllOnesValue(VT.getScalarSizeInBits()),
+                              DL, VT)
+            : TLI.getConstTrueVal(DAG, VT, DL);
+
     if (SDValue SCC = SimplifySelectCC(
+            DL, N0.getOperand(0), N0.getOperand(1), TrueVal,
----------------
chandlerc wrote:
> I'm confused....
> 
> Why wouldn't all this special casing of size one still be sunk into getConstTrueVal? I feel like I should understand this from your previous comment, but perhaps I'm being dense and don't....
We're combining a (sext (setcc (...)).

There are two options:
1) The setcc's type is i1. So the sext of the "true" value is all-1s, regardless of getBooleanContents().
2) The setcc's type is something else, say, an i8. In this case, it has already been extended - either zext, or sext, according to getBooleanContents(). So to sext it further, we want to splat the highest bit of the i8. For the "true" case, the result of this sext is the getBooleanContents-respecting true value of the right width.

I'll document this inline.


https://reviews.llvm.org/D22247