[TargetLowering] add special-case for demanded bits analysis of 'not'
We treat bitwise 'not' as a special operation and try not to reduce its all-ones mask.
Presumably, this is because a 'not' may be cheaper than a generic 'xor' or it may get
folded into another logic op if the target has those. However, if we can remove a logic
instruction by changing the xor's constant mask value, that should always be a win.
Note that the IR version of SimplifyDemandedBits() does not treat 'not' as a special-case
currently (although that's marked with a FIXME). So if you run this IR through -instcombine,
you should get the same end result. I'm hoping to add a different backend transform that
will expose this problem though, so I need to solve this first.
Differential Revision: https://reviews.llvm.org/D27356
llvm-svn: 288676
diff --git a/llvm/test/CodeGen/X86/not-and-simplify.ll b/llvm/test/CodeGen/X86/not-and-simplify.ll
index 3bee447..dfce6c6 100644
--- a/llvm/test/CodeGen/X86/not-and-simplify.ll
+++ b/llvm/test/CodeGen/X86/not-and-simplify.ll
@@ -5,20 +5,12 @@
; Clear high bits via shift, set them with xor (not), then mask them off.
define i32 @shrink_xor_constant1(i32 %x) {
-; NO_BMI-LABEL: shrink_xor_constant1:
-; NO_BMI: # BB#0:
-; NO_BMI-NEXT: shrl $31, %edi
-; NO_BMI-NEXT: notl %edi
-; NO_BMI-NEXT: andl $1, %edi
-; NO_BMI-NEXT: movl %edi, %eax
-; NO_BMI-NEXT: retq
-;
-; BMI-LABEL: shrink_xor_constant1:
-; BMI: # BB#0:
-; BMI-NEXT: shrl $31, %edi
-; BMI-NEXT: movl $1, %eax
-; BMI-NEXT: andnl %eax, %edi, %eax
-; BMI-NEXT: retq
+; ALL-LABEL: shrink_xor_constant1:
+; ALL: # BB#0:
+; ALL-NEXT: shrl $31, %edi
+; ALL-NEXT: xorl $1, %edi
+; ALL-NEXT: movl %edi, %eax
+; ALL-NEXT: retq
;
%sh = lshr i32 %x, 31
%not = xor i32 %sh, -1
@@ -32,8 +24,7 @@
; ALL-LABEL: shrink_xor_constant2:
; ALL: # BB#0:
; ALL-NEXT: shlb $5, %dil
-; ALL-NEXT: notb %dil
-; ALL-NEXT: andb $-32, %dil
+; ALL-NEXT: xorb $-32, %dil
; ALL-NEXT: movl %edi, %eax
; ALL-NEXT: retq
;