[llvm-dev] Unfolded additions of constants after promotion of @llvm.ctlz.i16 on SystemZ (original) (raw)

Sanjay Patel via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 8 13:23:12 PST 2019

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?

On Fri, Feb 8, 2019 at 10:20 AM Jonas Paulsson via llvm-dev < llvm-dev at lists.llvm.org> wrote:

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 = zeroextend 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::RETFLAG 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

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