[llvm-dev] InstCombine wrongful (?) optimization on BinOp with SameOperands

[Nicolas Brunie via llvm-dev]

Hi all,
     I have been looking at the way LLVM optimizes code before 
forwarding it to the backend I develop for my company and while building
define i32 @test_extract_subreg_func(i32 %x, i32 %y) #0 {
entry:
   %conv = zext i32 %x to i64
   %conv1 = zext i32 %y to i64
   %mul = mul nuw i64 %conv1, %conv
   %shr = lshr i64 %mul, 32
   %xor = xor i64 %shr, %mul
   %conv2 = trunc i64 %xor to i32
   ret i32 %conv2
}

I came upon the following optimization (during instcombine):
*IC: Visiting:   %mul = mul nuw i64 %conv, %conv1
IC: Visiting:   %shr = lshr i64 %mul, 32
IC: Visiting:   %conv2 = trunc i64 %shr to i32
IC: Visiting:   %conv3 = trunc i64 %mul to i32
IC: Visiting:   %xor = xor i32 %conv3, %conv2
IC: ADD:   %xor6 = xor i64 %mul, %shr
IC: Old =   %xor = xor i32 %conv3, %conv2
     New =   <badref> = trunc i64 %xor6 to i32
*
which seems to be performed by SDValue 
DAGCombiner::SimplifyBinOpWithSameOpcodeHands(SDNode *N)

In my backend's architecture truncate is free, but zext is not (and i64 
is not a desirable type for xor or any binary operation in general), so 
I would expect this optimization to be bypassed but because of the 
following statement   :
(N0.getOpcode() == ISD::TRUNCATE && (!TLI.isZExtFree(VT, Op0VT)  || 
!TLI.isTruncateFree(Op0VT, VT))
it is not (as isZExtFree return false for my architecture while 
isTruncateFree returns true). The comment on binop simplification says 
that binop over truncs should be optimize only if trunc is not free, so 
I do not understand the point of adding !isZExtFree at this point.
Can someone enlighten my ignorance on this optimization ?

best regards,
Nicolas Brunie
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