[flang-commits] [flang] [flang] Avoid undefined behaviour when parsing format expressions (PR #147539)

[LLVM Continuous Integration via flang-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `clang-aarch64-sve-vla` running on `linaro-g3-03` while building `flang` at step 6 "build stage 1".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/17/builds/9443

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (build stage 1) failure: 'ninja' (failure)
...
[3455/3458] Performing build step for 'runtimes'
[1/11] Generating Fortran dyndep file flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/Fortran.dd
[2/11] Building CXX object compiler-rt/lib/asan/CMakeFiles/RTAsan_dynamic_version_script_dummy.aarch64.dir/dummy.cpp.o
[3/11] Building CXX object compiler-rt/lib/hwasan/CMakeFiles/RTHwasan_dynamic_version_script_dummy.aarch64.dir/dummy.cpp.o
[4/11] Building CXX object compiler-rt/lib/ubsan/CMakeFiles/RTUbsan_dynamic_version_script_dummy.aarch64.dir/dummy.cpp.o
[5/11] Linking CXX shared library /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/lib/clang/21/lib/aarch64-unknown-linux-gnu/libclang_rt.hwasan.so
[6/11] Linking CXX shared library /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/lib/clang/21/lib/aarch64-unknown-linux-gnu/libclang_rt.ubsan_standalone.so
[7/11] Linking CXX shared library /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/lib/clang/21/lib/aarch64-unknown-linux-gnu/libclang_rt.asan.so
[8/11] Generating /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/compile_commands.json
[9/11] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/format.cpp.o
FAILED: flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/format.cpp.o 
/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/./bin/clang++ --target=aarch64-unknown-linux-gnu -D_DEBUG -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/llvm/flang-rt/include -I/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/llvm/flang-rt/../flang/include -I/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/runtimes/runtimes-bins/flang-rt -fPIC -fno-semantic-interposition -fvisibility-inlines-hidden -Werror=date-time -Werror=unguarded-availability-new -Wall -Wextra -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -Wimplicit-fallthrough -Wcovered-switch-default -Wno-noexcept-type -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -Wsuggest-override -Wstring-conversion -Wmisleading-indentation -Wctad-maybe-unsupported -fdiagnostics-color -ffunction-sections -fdata-sections -O3 -DNDEBUG -std=gnu++17 -UNDEBUG -fno-lto -fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables -U_GLIBCXX_ASSERTIONS -U_LIBCPP_ENABLE_ASSERTIONS -MD -MT flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/format.cpp.o -MF flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/format.cpp.o.d -o flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/format.cpp.o -c /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/llvm/flang-rt/lib/runtime/format.cpp
In file included from /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/llvm/flang-rt/lib/runtime/format.cpp:9:
In file included from /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/llvm/flang-rt/include/flang-rt/runtime/format-implementation.h:18:
/home/tcwg-buildbot/worker/clang-aarch64-sve-vla/llvm/flang/include/flang/Common/format.h:14:10: fatal error: 'llvm/Support/MathExtras.h' file not found
   14 | #include "llvm/Support/MathExtras.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~
1 error generated.
[10/11] Building CXX object flang-rt/lib/runtime/CMakeFiles/flang_rt.runtime.static.dir/time-intrinsic.cpp.o
ninja: build stopped: subcommand failed.
FAILED: runtimes/runtimes-stamps/runtimes-build /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/runtimes/runtimes-stamps/runtimes-build 
cd /home/tcwg-buildbot/worker/clang-aarch64-sve-vla/stage1/runtimes/runtimes-bins && /usr/local/bin/cmake --build .
ninja: build stopped: subcommand failed.

```

</details>

https://github.com/llvm/llvm-project/pull/147539