[llvm-bugs] [Bug 36433] New: LoopVectorizer calls getCastInstrCost for an illegal trunc instruction

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=36433

            Bug ID: 36433
           Summary: LoopVectorizer calls getCastInstrCost for an illegal
                    trunc instruction
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: paulsson at linux.vnet.ibm.com
                CC: llvm-bugs at lists.llvm.org

Created attachment 19895
  --> https://bugs.llvm.org/attachment.cgi?id=19895&action=edit
reduced testcase

getInstructionCost():

(gdb) p I->dump()
  %17 = trunc i32 %16 to i16

(gdb) p SrcVecTy->dump()
<2 x i32>

(gdb) p RetTy->dump()
i8

(gdb) p VectorTy->dump()
<2 x i8>

canTruncateToMinimalBitwidth(I, VF) returns true

(gdb) p SrcVecTy->dump()
<2 x i8>

(gdb) p VectorTy->dump()
<2 x i16>

TargetTransformInfo::getCastInstrCost() :
(gdb) p Dst->dump()
<2 x i16>

(gdb) p Src->dump()
<2 x i8>

These types do not make sense for a truncate.

->

lib/Target/SystemZ/SystemZTargetTransformInfo.cpp:520: unsigned int
llvm::SystemZTTIImpl::getVectorTruncCost(llvm::Type*, llvm::Type*): Assertion
`SrcTy->getPrimitiveSizeInBits() > DstTy->getPrimitiveSizeInBits() && "Packing
must reduce size of vector type."' failed.

I would hope that the SystemZ assert is correct, and that the loop vectorizer
should not have made this call.

---

Please excuse my simple patch for disabling instruction combining, which is how
this test case appeared with csmith.

I first tried to reproduce this with opt -debug-pass=Arguments and then using
opt with all the explicit passes without -O3, but that did not work for some
reason. First I got

'LLVM ERROR: Trying to construct TargetPassConfig without a target machine.
Scheduling a CodeGen pass without a target triple set?'

This is weird, because I passed the triple, and it's also in the input file.

When I removed the -targetpassconfig argument, I didn't get the error.

Also did not know how to do something like -stop-before=loop-vectorize.

So, to rerun (with the patch for -disable-instcombine applied):

bin/opt -O3 -mcpu=z13 -o out.opt.ll tc_loopvec_trunc.ll
-enable-loop-versioning-licm -disable-instcombine -enable-tbaa=false

BTW, I see a *lot* of errors when I use my disable-instcombine option. Given
this, I feel like I must ask if instcombine is truly optional, or if other
passes depend on it? Before rewriting instcombine per some discussion I read a
while back ago, I hope people are aware of this...

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