[X86] BEXTR SimplifyDemandedBitsForTargetNode - length == 0 -> result = 0

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index ebb8545..2a3ad34 100644 (file)
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -36290,6 +36290,11 @@ bool X86TargetLowering::SimplifyDemandedBitsForTargetNode(
     if (SimplifyDemandedBits(Op1, DemandedMask, Known1, TLO, Depth + 1))
       return true;
 
+    // If the length is 0, replace with 0.
+    KnownBits LengthBits = Known1.extractBits(8, 8);
+    if (LengthBits.isZero())
+      return TLO.CombineTo(Op, TLO.DAG.getConstant(0, SDLoc(Op), VT));
+
     break;
   }
   }

diff --git a/llvm/test/CodeGen/X86/combine-bextr.ll b/llvm/test/CodeGen/X86/combine-bextr.ll
index d4763b3..852e96e 100644 (file)
--- a/llvm/test/CodeGen/X86/combine-bextr.ll
+++ b/llvm/test/CodeGen/X86/combine-bextr.ll
@@ -13,8 +13,7 @@ define i32 @bextr_zero_length(i32 %x, i32 %y) {
 ;
 ; X64-LABEL: bextr_zero_length:
 ; X64:       # %bb.0:
-; X64-NEXT:    movzbl %sil, %eax
-; X64-NEXT:    bextrl %eax, %edi, %eax
+; X64-NEXT:    xorl %eax, %eax
 ; X64-NEXT:    retq
   %1 = and i32 %y, 255
   %2 = tail call i32 @llvm.x86.bmi.bextr.32(i32 %x, i32 %1)