Syntax error in inline assembly when targeting armv7

[j-jorge]

Hello, I encounter an issue when compiling the armeabi-v7a library of my Android project. The problem can be reproduced by compiling a file with this content:

// test.cpp
#include <boost/unordered/unordered_flat_map.hpp>

The compilation fails with the following message:

<inline asm>:1:41: error: expected '%<type>' or "<type>"
    1 | .pushsection ".debug_gdb_scripts", "MS",@progbits,1
      |                                         ^
<inline asm>:318:12: error: .popsection without corresponding .pushsection
  318 | .popsection
      |            ^
2 errors generated.

The command used to compile the file is:

path/to/ndk/27.2.12479018/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ \
  --target=armv7-none-linux-androideabi21 \
  --sysroot=path/to/ndk/27.2.12479018/toolchains/llvm/prebuilt/linux-x86_64/sysroot \
  -Ipath/to/include/ \
  -fvisibility=hidden \
  -fdata-sections \
  -ffunction-sections \
  -funwind-tables \
  -fstack-protector-strong \
  -no-canonical-prefixes \
  -D_FORTIFY_SOURCE=2 \
  -march=armv7-a \
  -mthumb \
  -fexceptions \
  -frtti \
  -stdlib=libc++ \
  -g \
  -fno-limit-debug-info \
  -std=c++20 \
  -fPIC \
  -fcolor-diagnostics -pthread \
  -MD \
  -MT \
  test.o \
  -c test.cpp

Boost 1.87.0 is installed in the path/to/include directory passed to the -I option. There is no problem with the arm64 nor the x86_64 architectures.

As a workaround I can compile with -DBOOST_ALL_NO_EMBEDDED_GDB_SCRIPTS but I which I did not have to do that (i.e. that the inline assembly would be omitted when the target does not support it).

Note that it happens here with Boost.Unordered but I expect a similar problem with Boost.Interprocess and Boost.Json as they also use this GDB macro :)

[grisumbras]

Related llvm/llvm-project#120871

[grisumbras]

Looking at the workaround linked in that issue, another option is to use -fno-integrated-as to use system assembler rather than the integrated Clang assembler.

[grisumbras]

Apparently, this fixes it: ned14/outcome#308.

[grisumbras]

Note on targets where the @ character is the start of a comment (eg ARM) then another character is used instead. For example the ARM port uses the % character.

(https://www.sourceware.org/binutils/docs/as/Section.html)

So, this is actually the expected behaviour. Interestingly, I don't see universal support for % as section type prefix documented, but it appears to be the case. I tested a few targets on CE.