Extended Description

https://reviews.llvm.org/D93779 (landed as 4914299) changes ADT/SmallVector.h, which makes clang smart enough to diagnose a -Wsometimes-uninitialized for a MUL value passed to a SmallVector in AArch64InstrInfo.cpp. This build warning (or error when -Werror is enabled) is fixed by always initializing to nullptr in 752fafd, but if we add an assertion that MUL is not nullptr when we add it to the SmallVector, we get an assertion failure in two tests:

$ ninja check-llvm-codegen-aarch64
FAIL: LLVM :: CodeGen/AArch64/tailmerging_in_mbp.ll (1886 of 1908)
******************** TEST 'LLVM :: CodeGen/AArch64/tailmerging_in_mbp.ll' FAILED ********************
Script:

: 'RUN: at line 1'; /home/rupprecht/src/llvm-build/dev/bin/llc </home/rupprecht/src/llvm-project/llvm/test/CodeGen/AArch64/tailmerging_in_mbp.ll -mtriple=aarch64-eabi -verify-machine-dom-info | /home/rupprecht/src/llvm-build/dev/bin/FileCheck --allow-unused-prefixes=false /home/rupprecht/src/llvm-project/llvm/test/CodeGen/AArch64/tailmerging_in_mbp.ll

Exit Code: 2

Command Output (stderr):

llc: /home/rupprecht/src/llvm-project/llvm/lib/Target/AArch64/AArch64InstrInfo.cpp:5697: virtual void llvm::AArch64InstrInfo::genAlternativeCodeSequence(llvm::MachineInstr &, llvm::MachineCombinerPattern, SmallVectorImpl<llvm::MachineInstr *> &, SmallVectorImpl<llvm::MachineInstr *> &, DenseMap<unsigned int, unsigned int> &) const: Assertion `MUL && "MUL was never set"' failed.
PLEASE submit a bug report to https://bugs.llvm.org/ and include the crash backtrace.
Stack dump:
0. Program arguments: /home/rupprecht/src/llvm-build/dev/bin/llc -mtriple=aarch64-eabi -verify-machine-dom-info

1.  Running pass 'Function Pass Manager' on module '<stdin>'.
   

2.  Running pass 'Machine InstCombiner' on function '@test'
   

...
Failed Tests (2):
LLVM :: CodeGen/AArch64/tailmerging_in_mbp.ll
LLVM :: CodeGen/AArch64/urem-seteq-nonzero.ll

The assertion is already added to llvm/lib/Target/AArch64/AArch64InstrInfo.cpp at the end of AArch64InstrInfo::genAlternativeCodeSequence(), but commented out. Uncomment to reproduce the above.