Enable Google Benchmark to run in WebAssembly with filesystem disabled

[XueSongTap]

Problem Description

When Google Benchmark runs in a WebAssembly environment, especially when compiled with Emscripten's -s FILESYSTEM=0 flag (which disables filesystem support), the program aborts when trying to access /sys/devices/system/cpu/cpu0/cache/ to retrieve CPU cache information.

This happens because the current implementation defaults to calling GetCacheSizesFromKVFS() on non-Windows/macOS/QNX/QURT platforms, a function that relies on filesystem access, which is not available in WebAssembly when the filesystem is disabled.

Solution

This PR modifies the GetCacheSizes() function in src/sysinfo.cc to detect Emscripten environments and return an empty cache info vector when running in WebAssembly, avoiding access to a non-existent filesystem.

The change is minimal, simply adding detection for the EMSCRIPTEN macro and merging it with the QURT OS handling, as both return empty cache information.

Related Issue

Fixes #1952

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[XueSongTap]

self test:

yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_with_fs.js -I../include -L./src -lbenchmark
cache:INFO: generating system asset: symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json" for subsequent builds)
cache:INFO:  - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_without_fs.js -I../include -L./src -lbenchmark -s FILESYSTEM=0
cache:INFO: generating system asset: symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json" for subsequent builds)
cache:INFO:  - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_without_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:43+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_without_fs.js
Run on (1 X 999.986 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark             Time             CPU   Iterations
-------------------------------------------------------
BM_EmptyLoop      0.000 ns        0.000 ns   1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_with_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:57+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_with_fs.js
Run on (1 X 999.99 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark             Time             CPU   Iterations
-------------------------------------------------------
BM_EmptyLoop      0.000 ns        0.000 ns   1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ cat test_benchmark.cpp
#include <benchmark/benchmark.h>

static void BM_EmptyLoop(benchmark::State& state) {
  for (auto _ : state) {

  }
}
BENCHMARK(BM_EmptyLoop);

BENCHMARK_MAIN();yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$

[XueSongTap]

sek test

yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_with_fs.js -I../include -L./src -lbenchmark
cache:INFO: generating system asset: symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/4ecc501da4e0b22aa538e28f5ac2b2f25c24ad56.json" for subsequent builds)
cache:INFO:  - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ emcc -O2 test_benchmark.cpp -o test_without_fs.js -I../include -L./src -lbenchmark -s FILESYSTEM=0
cache:INFO: generating system asset: symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json... (this will be cached in "/home/yxc/code/2503/emsdk/upstream/emscripten/cache/symbol_lists/9e2ecc3f9ddd9359bfd85a70e497ee587e11349b.json" for subsequent builds)
cache:INFO:  - ok
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_without_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:43+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_without_fs.js
Run on (1 X 999.986 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark             Time             CPU   Iterations
-------------------------------------------------------
BM_EmptyLoop      0.000 ns        0.000 ns   1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ node test_with_fs.js
***WARNING*** Failed to set thread affinity. Estimated CPU frequency may be incorrect.
2025-03-25T00:08:57+08:00
Running /home/yxc/code/2503/benchmark_yxc/build_em/test_with_fs.js
Run on (1 X 999.99 MHz CPU )
***WARNING*** Library was built as DEBUG. Timings may be affected.
-------------------------------------------------------
Benchmark             Time             CPU   Iterations
-------------------------------------------------------
BM_EmptyLoop      0.000 ns        0.000 ns   1000000000000
yxc@yxc-MS-7B89:~/code/2503/benchmark_yxc/build_em$ cat test_benchmark.cpp
#include <benchmark/benchmark.h>

static void BM_EmptyLoop(benchmark::State& state) {
  for (auto _ : state) {

  }
}
BENCHMARK(BM_EmptyLoop);

BENCHMARK_MAIN();

[LebedevRI]

This does seem reasonable, but i have a question: is there any way to know
if the code is being compiled with filesystem support or not?
Can you dump the macros that the compiler sets in both modes, and post here?

https://stackoverflow.com/questions/2224334/gcc-dump-preprocessor-defines
https://stackoverflow.com/questions/4548702/whats-the-equivalent-of-cpp-dd-for-clang

[XueSongTap]

@LebedevRI I've investigated the preprocessor macros between builds with and without filesystem support:

yxc@yxc-MS-7B89:~/code/2503/emsdk$ echo | emcc -dM -E - > with_fs.txt
yxc@yxc-MS-7B89:~/code/2503/emsdk$ echo | emcc -dM -E -s FILESYSTEM=0 - > without_fs.txt
emcc: warning: linker setting ignored during compilation: 'FILESYSTEM' [-Wunused-command-line-argument]
yxc@yxc-MS-7B89:~/code/2503/emsdk$ diff with_fs.txt without_fs.txt
yxc@yxc-MS-7B89:~/code/2503/emsdk$

I received this warning: emcc: warning: linker setting ignored during compilation: 'FILESYSTEM' [-Wunused-command-line-argument]

This confirms that -s FILESYSTEM=0 is a linker option rather than a preprocessor definition. During preprocessing, this option is ignored, so there's no specific preprocessor macro that can be used to detect filesystem support.

Therefore, using #ifdef EMSCRIPTEN appears to be the most reliable approach, as there's no more specific macro available to detect filesystem support status. The filesystem options are handled at link time rather than compile time in Emscripten.

[LebedevRI]

That's unfortunate. Is there some emscripten-native way to query CPU cache properties?