[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

1 files changed, 7 insertions, 16 deletions

diff --git a/llvm/test/CodeGen/X86/not-and-simplify.ll b/llvm/test/CodeGen/X86/not-and-simplify.ll
index 3bee4472c14..dfce6c68150 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 @@ define i8 @shrink_xor_constant2(i8 %x) {
 ; 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
 ;