[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 `premerge-monolithic-linux` running on `premerge-linux-1` while building `flang` at step 6 "build-unified-tree".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/153/builds/37499

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

```
Step 6 (build-unified-tree) failure: build (failure)
...
0.023 [3/8/4] Building CXX object compiler-rt/lib/nsan/CMakeFiles/RTNsan_dynamic_version_script_dummy.x86_64.dir/dummy.cpp.o
0.023 [2/8/5] Building CXX object compiler-rt/lib/memprof/CMakeFiles/RTMemprof_dynamic_version_script_dummy.x86_64.dir/dummy.cpp.o
0.066 [1/8/6] Linking CXX shared library /build/buildbot/premerge-monolithic-linux/build/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.ubsan_standalone.so
0.067 [1/7/7] Linking CXX shared library /build/buildbot/premerge-monolithic-linux/build/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.nsan.so
0.068 [1/6/8] Linking CXX shared library /build/buildbot/premerge-monolithic-linux/build/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.hwasan_aliases.so
0.070 [1/5/9] Linking CXX shared library /build/buildbot/premerge-monolithic-linux/build/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.hwasan.so
0.074 [1/4/10] Linking CXX shared library /build/buildbot/premerge-monolithic-linux/build/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.memprof.so
0.081 [1/3/11] Linking CXX shared library /build/buildbot/premerge-monolithic-linux/build/lib/clang/21/lib/x86_64-unknown-linux-gnu/libclang_rt.asan.so
0.570 [1/2/12] Generating /build/buildbot/premerge-monolithic-linux/build/compile_commands.json
1.025 [1/1/13] 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 
/build/buildbot/premerge-monolithic-linux/build/./bin/clang++ --target=x86_64-unknown-linux-gnu -D_DEBUG -D_GLIBCXX_ASSERTIONS -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/build/buildbot/premerge-monolithic-linux/llvm-project/flang-rt/include -I/build/buildbot/premerge-monolithic-linux/llvm-project/flang-rt/../flang/include -I/build/buildbot/premerge-monolithic-linux/build/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 -UNDEBUG -fno-lto -fno-exceptions -fno-rtti -funwind-tables -fno-asynchronous-unwind-tables -U_GLIBCXX_ASSERTIONS -U_LIBCPP_ENABLE_ASSERTIONS -std=gnu++17 -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 /build/buildbot/premerge-monolithic-linux/llvm-project/flang-rt/lib/runtime/format.cpp
In file included from /build/buildbot/premerge-monolithic-linux/llvm-project/flang-rt/lib/runtime/format.cpp:9:
In file included from /build/buildbot/premerge-monolithic-linux/llvm-project/flang-rt/include/flang-rt/runtime/format-implementation.h:18:
/build/buildbot/premerge-monolithic-linux/llvm-project/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.
ninja: build stopped: subcommand failed.
FAILED: runtimes/runtimes-stamps/runtimes-build /build/buildbot/premerge-monolithic-linux/build/runtimes/runtimes-stamps/runtimes-build 
cd /build/buildbot/premerge-monolithic-linux/build/runtimes/runtimes-bins && /etc/cmake/bin/cmake --build .
ninja: build stopped: subcommand failed.

```

</details>

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