[llvm] 8c0e715 - [X86] BEXTR SimplifyDemandedBitsForTargetNode - length == 0 -> result = 0

[Simon Pilgrim via llvm-commits]

Author: Simon Pilgrim
Date: 2020-02-03T16:50:03Z
New Revision: 8c0e715eb281fa8c3b1605191294671eb6a898b5

URL: https://github.com/llvm/llvm-project/commit/8c0e715eb281fa8c3b1605191294671eb6a898b5
DIFF: https://github.com/llvm/llvm-project/commit/8c0e715eb281fa8c3b1605191294671eb6a898b5.diff

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

Added: 
    

Modified: 
    llvm/lib/Target/X86/X86ISelLowering.cpp
    llvm/test/CodeGen/X86/combine-bextr.ll

Removed: 
    


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diff  --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index ebb854511c86..2a3ad344973e 100644
--- 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 d4763b3f076c..852e96ebebdd 100644
--- 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)