[Legalizer] Don't throw away false low half for GT/LT SETCC

[mkuper]

Summary

When expanding a SETCC for which the low half is known to evaluate to false, we can only throw it away for LT/GT comparisons, not LE/GE.

This fixes PR29170.

[spatel]

LGTM - but there's a bug in the regression test that needs to be fixed.

test/CodeGen/X86/pr29170.ll

10 ↗

(On Diff #70050)

'CHECL' - use utils/update_test_checks.py to avoid human error? :)

[mkuper]

test/CodeGen/X86/pr29170.ll

10 ↗

(On Diff #70050)

Yikes, thanks!
(I intentionally didn't use update_test_checks, since I wanted a fairly "loose" test. But should have double checked the checks!)

llvm/trunk/lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp

HiCmp = TLI.SimplifySetCC(getSetCCResultType(LHSHi.getValueType()), LHSHi,
                               RHSHi, CCCode, false, DagCombineInfo, dl);
   if (!HiCmp.getNode())
     HiCmp =
         DAG.getNode(ISD::SETCC, dl, getSetCCResultType(LHSHi.getValueType()),
                     LHSHi, RHSHi, DAG.getCondCode(CCCode));
 
   ConstantSDNode *LoCmpC = dyn_cast<ConstantSDNode>(LoCmp.getNode());
   ConstantSDNode *HiCmpC = dyn_cast<ConstantSDNode>(HiCmp.getNode());
-  if ((LoCmpC && LoCmpC->isNullValue()) ||
-      (HiCmpC && HiCmpC->isNullValue() &&
-       (CCCode == ISD::SETLE || CCCode == ISD::SETGE || CCCode == ISD::SETUGE ||
-        CCCode == ISD::SETULE)) ||
-      (HiCmpC && HiCmpC->getAPIntValue() == 1 &&
-       (CCCode == ISD::SETLT || CCCode == ISD::SETGT || CCCode == ISD::SETUGT ||
-        CCCode == ISD::SETULT))) {
-    // low part is known false, returns high part.
+
+  bool EqAllowed = (CCCode == ISD::SETLE || CCCode == ISD::SETGE ||
+                    CCCode == ISD::SETUGE || CCCode == ISD::SETULE);
+
+  if ((EqAllowed && (HiCmpC && HiCmpC->isNullValue())) ||
+      (!EqAllowed && ((HiCmpC && (HiCmpC->getAPIntValue() == 1)) ||
+                      (LoCmpC && LoCmpC->isNullValue())))) {
     // For LE / GE, if high part is known false, ignore the low part.
-    // For LT / GT, if high part is known true, ignore the low part.
+    // For LT / GT: if low part is known false, return the high part.
+    //              if high part is known true, ignore the low part.
     NewLHS = HiCmp;
     NewRHS = SDValue();
     return;
   }
 
   if (LHSHi == RHSHi) {
     // Comparing the low bits is enough.
     NewLHS = LoCmp;

llvm/trunk/test/CodeGen/X86/pr29170.ll

+; RUN: llc < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-f64:32:64-f80:32-n8:16:32-S128"
+target triple = "i386-unknown-linux-gnu"
+
+@b = global i16 0, align 4
+
+; CHECK-LABEL: @main
+; CHECK: cmpl
+; CHECK: sbbl
+define i32 @main() {
+entry:
+  %true = icmp eq i32 0, 0
+  %const = bitcast i64 -4294967296 to i64
+  br i1 %true, label %go, label %if.else
+
+go:
+  %b = load i16, i16* @b, align 4
+  %sext = shl i16 %b, 8
+  %conv = ashr i16 %sext, 8
+  %neg4 = xor i64 %const, -1
+  %conv5 = zext i16 %conv to i64
+  %cmp = icmp slt i64 %conv5, %neg4
+  br i1 %cmp, label %if.then, label %if.else
+
+if.then:
+  ret i32 42
+
+if.else:
+  ret i32 0
+}
+