[PATCH] D111804: [InstCombine] Don't combine CmpInst that used for verify the legality of the lshr

[Tiehu Zhang via Phabricator via llvm-commits]

TiehuZhang created this revision.
TiehuZhang added reviewers: spatel, dmgreen, sdesmalen, mdchen.
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The cmpInst used to identify the shift operand legalilty before the lshr is necessary and cannot be combined and eliminated. Otherwise, an unknown error may occur.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D111804

Files:
  llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  llvm/test/Transforms/InstCombine/shift.ll


Index: llvm/test/Transforms/InstCombine/shift.ll
===================================================================
--- llvm/test/Transforms/InstCombine/shift.ll
+++ llvm/test/Transforms/InstCombine/shift.ll
@@ -1851,3 +1851,31 @@
   %r = and i8 %s, 7  ; 0b00000111
   ret i8 %r
 }
+
+define i32 @cmp_lshr_cmp(i32 %0) {
+; CHECK-LABEL: @cmp_lshr_cmp(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i32 [[TMP0:%.*]], 0
+; CHECK-NEXT:    [[SHR:%.*]] = lshr i32 7, [[TMP0]]
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp slt i32 [[SHR]], [[TMP0]]
+; CHECK-NEXT:    [[OR_COND:%.*]] = or i1 [[CMP]], [[CMP1]]
+; CHECK-NEXT:    br i1 [[OR_COND]], label [[COND_END:%.*]], label [[COND_FALSE:%.*]]
+; CHECK:       cond.false:
+; CHECK-NEXT:    ret i32 1
+; CHECK:       cond.end:
+; CHECK-NEXT:    ret i32 0
+;
+entry:
+  %cmp = icmp slt i32 %0, 0
+  %shr = lshr i32 7, %0
+  %cmp1 = icmp sgt i32 %0, %shr
+  %or.cond = or i1 %cmp, %cmp1
+  br i1 %or.cond, label %cond.end, label %cond.false
+
+cond.false:                                       ; preds = %entry
+  ret i32 1
+
+cond.end:                                         ; preds = %entry, %cond.false
+  ret i32 0
+}
+
Index: llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -728,6 +728,12 @@
     default: return nullptr;
   }
 
+  // If x > -1 or x >= 0 is used to verify the legality of lshr,
+  // it shouldn't be combined.
+  if (auto *I = dyn_cast<Operator>(RangeEnd))
+    if (I->getOpcode() == Instruction::LShr)
+      return nullptr;
+
   // This simplification is only valid if the upper range is not negative.
   KnownBits Known = computeKnownBits(RangeEnd, /*Depth=*/0, Cmp1);
   if (!Known.isNonNegative())


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