Unfolded additions of constants after promotion of @llvm.ctlz.i16 on SystemZ

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#1

Hi,

SystemZ supports @llvm.ctlz.i64() natively with a single instruction (FLOGR), and lesser bitwidth versions of the intrinsic are promoted to i64.

For some reason, this leads to unfolded additions of constants as shown below:

This function:

define i16 @fun(i16 %arg) {
%1 = tail call i16 @llvm.ctlz.i16(i16 %arg, i1 false)
ret i16 %1
}

,gives this optimized DAG as input to instruction selection:

SelectionDAG has 15 nodes:
t0: ch = EntryToken
t2: i32,ch = CopyFromReg t0, Register:i32 %0
t10: i32 = and t2, Constant:i32<65535>
t16: i64 = zero_extend t10
t17: i64 = ctlz t16
t22: i64 = add t17, Constant:i64<-32>
t20: i32 = truncate t22
t15: i32 = add t20, Constant:i32<-16>
t7: ch,glue = CopyToReg t0, Register:i32 $r2l, t15
t8: ch = SystemZISD::RET_FLAG t7, Register:i32 $r2l, t7:1

It seems that SelectionDAG::computeKnownBits() has a case for ISD::CTLZ, and it seems to figure out that the high bits of t17 are zero, as expected.

t17 is guaranteed to have a value between 48 and 64, so there could not be any overflow here, even though I am not sure if that's the problem or not... Should DAGCombiner::visitADD() handle this, or perhaps visitTRUNCATE()?

Thanks for any help,

Jonas

#2

If I’m seeing it correctly, (part of?) the fold you’re looking for is here:
https://reviews.llvm.org/rL350006

…but it’s restricted to pre-legalization.

I don’t remember exactly what the problem was allowing that fold post-legalization, but maybe you can loosen that restriction?

#3

Hi sanjay,

If I’m seeing it correctly, (part of?) the fold you’re looking for is here:
https://reviews.llvm.org/rL350006

…but it’s restricted to pre-legalization.

I don’t remember exactly what the problem was allowing that fold post-legalization, but maybe you can loosen that restriction?

Thanks! I tried just to remove the !LegalOperations condition (DAGCombiner.cpp:10056), and indeed my problem was solved. Doing this on SystemZ (for all of the opcodes) did not affect SPEC that much. Opcode counts (trunk to left):

aghi : 38759 38742 -17
ahi : 34921 34936 +15
risbgn : 37104 37092 -12
nill : 2172 2183 +11
lr : 29731 29735 +4
sr : 6055 6059 +4
srk : 3743 3741 -2
lhi : 89566 89568 +2
risblg : 6528 6529 +1
la : 192375 192374 -1

Spill|Reload : 189670 189670 +0

So, to me it seems this could be the default on SystemZ at least.

/Jonas
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