[llvm-commits] patch: folding (zext (and x, cst))

[Nick Lewycky]

I have an unfinished patch. I was looking to optimize:

   define i32 @test1(i8 %x) nounwind readnone {
     %A = and i8 %x, -32
     %B = zext i8 %A to i32
     ret i32 %B
   }

which currently does a mov into %al, then the "and", then extends, into 
doing a single extending mov, then an "and" in 32-bits. The rule I 
decided upon is "(zext (and x, cst)) -> (and (anyext x), (zext cst))" in 
the DAG combiner.

That wasn't so hard to implement, but I haven't been able to chase down 
all of the fallout. The primary problem is that any-extends are showing 
up in places that didn't expect them. I added another DAG combine to 
fold away an anyext in one transform with sbbl, but then got rather stuck.

The patterns added in this commit: 
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20110502/120547.html
isn't matching any more. The relevant portion of before:

         0x90580f0: i32 = X86ISD::CMP 0x9057a90, 0x9057a08
       0x9057f58: i32 = X86ISD::SETCC_CARRY 0x9057fe0, 0x90580f0
     0x9058068: i32 = sub 0x9057a90, 0x9057f58

and after:

           0x96ff548: i32 = X86ISD::CMP 0x96feee8, 0x96fee60
         0x96ff4c0: i8 = X86ISD::SETCC_CARRY 0x96ff438, 0x96ff548
       0x96fef70: i32 = sign_extend 0x96ff4c0
     0x96ff080: i32 = sub 0x96feee8, 0x96fef70

I'm not sure why this isn't being folded away, because 
X86InstrCompiler.td has a pattern that matches anyext(X86setcc_c) and 
turns it into a single X86setcc_c of the extended type. Why isn't that 
firing here?

I've attached my patch and the result of running 'llc -debug 
test/CodeGen/X86/add-of-carry.ll' with my patch in place.

Nick
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