[llvm] [AArch64] Avoid selecting XAR for reverse operations. (PR #178706)

[via llvm-commits]

github-actions[bot] wrote:

<!--PREMERGE ADVISOR COMMENT: Linux-->
# :penguin: Linux x64 Test Results

* 3105 tests passed
* 7 tests skipped

All executed tests passed, but another part of the build **failed**. Click on a failure below to see the details.

<details>
<summary>lib/Target/AArch64/CMakeFiles/LLVMAArch64CodeGen.dir/AArch64ISelLowering.cpp.o</summary>

```
FAILED: lib/Target/AArch64/CMakeFiles/LLVMAArch64CodeGen.dir/AArch64ISelLowering.cpp.o
sccache /opt/llvm/bin/clang++ -D_DEBUG -D_GLIBCXX_ASSERTIONS -D_GLIBCXX_USE_CXX11_ABI=1 -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/lib/Target/AArch64 -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64 -I/home/gha/actions-runner/_work/llvm-project/llvm-project/build/include -I/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/include -gmlt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wc++98-compat-extra-semi -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wno-pass-failed -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=c++17 -fvisibility=hidden -UNDEBUG -fno-exceptions -funwind-tables -fno-rtti -MD -MT lib/Target/AArch64/CMakeFiles/LLVMAArch64CodeGen.dir/AArch64ISelLowering.cpp.o -MF lib/Target/AArch64/CMakeFiles/LLVMAArch64CodeGen.dir/AArch64ISelLowering.cpp.o.d -o lib/Target/AArch64/CMakeFiles/LLVMAArch64CodeGen.dir/AArch64ISelLowering.cpp.o -c /home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20863:14: error: variable 'RevOp' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
20863 |   } else if (EltSize == 64) {
|              ^~~~~~~~~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20869:34: note: uninitialized use occurs here
20869 |                      DAG.getNode(RevOp, DL, HalfVT, N0->getOperand(0)));
|                                  ^~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20863:10: note: remove the 'if' if its condition is always true
20863 |   } else if (EltSize == 64) {
|          ^~~~~~~~~~~~~~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20859:17: note: initialize the variable 'RevOp' to silence this warning
20859 |   unsigned RevOp;
|                 ^
|                  = 0
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20828:14: error: variable 'RevOp' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
20828 |     else if (EltSize == 64)
|              ^~~~~~~~~~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20831:24: note: uninitialized use occurs here
20831 |     return DAG.getNode(RevOp, DL, VT, N0.getOperand(0), N0.getOperand(1),
|                        ^~~~~
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20828:10: note: remove the 'if' if its condition is always true
20828 |     else if (EltSize == 64)
|          ^~~~~~~~~~~~~~~~~~
20829 |       RevOp = AArch64ISD::REVW_MERGE_PASSTHRU;
/home/gha/actions-runner/_work/llvm-project/llvm-project/llvm/lib/Target/AArch64/AArch64ISelLowering.cpp:20823:19: note: initialize the variable 'RevOp' to silence this warning
20823 |     unsigned RevOp;
|                   ^
|                    = 0
2 errors generated.
```
</details>

If these failures are unrelated to your changes (for example tests are broken or flaky at HEAD), please open an issue at https://github.com/llvm/llvm-project/issues and add the `infrastructure` label.

https://github.com/llvm/llvm-project/pull/178706