key_selector: compile with clang and -fno-exceptions by lemire · Pull Request #2913 · simdjson/simdjson

lemire · 2026-10-06T21:23:22Z

simdjson 5.0.2 does not compile with clang in C++20 mode when exceptions are disabled (-fno-exceptions), which is how Node.js and Chromium build:

simdjson.h:81978:5: error: cannot use 'throw' with exceptions disabled

The compile-time perfect-hash construction in key_selector.h reports failures with throw "..." inside consteval functions. Clang rejects any throw expression when exceptions are disabled, even one that is only evaluated at compile time. GCC accepts it.

Changes

key_selector.h: report the failures by calling a non-constexpr function instead of throwing. Reaching it during constant evaluation is still a compile-time error, and the diagnostic shows the call with its message (e.g. duplicate keys in for_each<"a", "a"> still fail to compile, with or without exceptions).
cmake/developer-options.cmake: compile the tests with -fno-exceptions when SIMDJSON_EXCEPTIONS=OFF.

Why CI did not catch it

In cmake/exception-flags.cmake, -fno-exceptions is only added when CMAKE_COMPILER_IS_GNUCC, only through target_link_libraries, and only to the library target. So the "CXX 20, noexcept" jobs (g++-13 and clang++-16) never compiled the headers without exceptions. With the CMake change alone, ondemand_object_find_field_tests fails to build on master with the error above.

I did not make the library itself build with -fno-exceptions: with libc++, std::string::__throw_length_error then calls abort(), which breaks the avoid_abort test.

Testing

AppleClang, -DSIMDJSON_CXX_STANDARD=20 -DSIMDJSON_EXCEPTIONS=OFF -DSIMDJSON_DEVELOPER_MODE=ON: everything builds, 109/109 tests pass. Node.js main also builds with simdjson 5.0.2 plus this patch.

The compile-time perfect-hash construction reported failures with `throw "..."` inside consteval functions. Clang rejects any throw expression when exceptions are disabled, even one that is only evaluated at compile time, so simdjson.h no longer compiled under C++20 with -fno-exceptions (as used by Node.js and Chromium). GCC accepts it, which is why it went unnoticed. Report the failures by calling a non-constexpr function instead: it is still a compile-time error, with a diagnostic pointing at the message, and it needs no exception support. Also compile the tests with -fno-exceptions when SIMDJSON_EXCEPTIONS is OFF. Until now, only GCC got the flag, and only on the library's link line, so the "noexcept" CI jobs never compiled the headers without exceptions.

PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

env.h includes inspector_profiler.h, which included simdjson.h for the types of two virtual methods and a parser member. As a result, about 155 files in src/ parsed the amalgamated simdjson header, which is over 10 MB. Keep simdjson out of inspector_profiler.h: the profile connections now say which field of `result` holds the profile, and WriteProfile() takes the profile as raw JSON. The parser becomes a local variable, so its buffers are no longer kept alive for the lifetime of the Environment. Also drop the unused include from node_task_runner.h, and let the config file reader rewind its document instead of parsing the whole file a second time with a second parser. Now only the eight files that use simdjson include it. On an Apple M-series machine, a clean rebuild of the src/ objects takes about 15% less CPU time (376 s to 320 s) and 19% less wall time at -j14. Signed-off-by: Daniel Lemire <[email protected]> PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

env.h includes inspector_profiler.h, which included simdjson.h for the types of two virtual methods and a parser member. As a result, about 155 files in src/ parsed the amalgamated simdjson header, which is over 10 MB. Keep simdjson out of inspector_profiler.h: the profile connections now say which field of `result` holds the profile, and WriteProfile() takes the profile as raw JSON. The parser becomes a local variable, so its buffers are no longer kept alive for the lifetime of the Environment. Also drop the unused include from node_task_runner.h, and let the config file reader rewind its document instead of parsing the whole file a second time with a second parser. Now only the eight files that use simdjson include it. On an Apple M-series machine, a clean rebuild of the src/ objects takes about 15% less CPU time (376 s to 320 s) and 19% less wall time at -j14. Signed-off-by: Daniel Lemire <[email protected]> PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

env.h includes inspector_profiler.h, which included simdjson.h for the types of two virtual methods and a parser member. As a result, about 155 files in src/ parsed the amalgamated simdjson header, which is over 10 MB. Keep simdjson out of inspector_profiler.h: the profile connections now say which field of `result` holds the profile, and WriteProfile() takes the profile as raw JSON. The parser becomes a local variable, so its buffers are no longer kept alive for the lifetime of the Environment. Also drop the unused include from node_task_runner.h, and let the config file reader rewind its document instead of parsing the whole file a second time with a second parser. Now only the eight files that use simdjson include it. On an Apple M-series machine, a clean rebuild of the src/ objects takes about 15% less CPU time (376 s to 320 s) and 19% less wall time at -j14. Signed-off-by: Daniel Lemire <[email protected]> PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

env.h includes inspector_profiler.h, which included simdjson.h for the types of two virtual methods and a parser member. As a result, about 155 files in src/ parsed the amalgamated simdjson header, which is over 10 MB. Keep simdjson out of inspector_profiler.h: the profile connections now say which field of `result` holds the profile, and WriteProfile() takes the profile as raw JSON. The parser becomes a local variable, so its buffers are no longer kept alive for the lifetime of the Environment. Also drop the unused include from node_task_runner.h, and let the config file reader rewind its document instead of parsing the whole file a second time with a second parser. Now only the eight files that use simdjson include it. On an Apple M-series machine, a clean rebuild of the src/ objects takes about 15% less CPU time (376 s to 320 s) and 19% less wall time at -j14. Signed-off-by: Daniel Lemire <[email protected]> PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

PR-URL: #66620 Refs: simdjson/simdjson#2913 Refs: #66495 Reviewed-By: Yagiz Nizipli <[email protected]> Reviewed-By: Antoine du Hamel <[email protected]> Reviewed-By: Richard Lau <[email protected]>

lemire added the LGTM label Oct 6, 2026

lemire merged commit e9cdb87 into master Oct 6, 2026
98 of 100 checks passed

lemire deleted the fix-key-selector-no-exceptions branch October 6, 2026 23:24

lemire mentioned this pull request Oct 9, 2026

deps,src: update simdjson and trim its includes nodejs/node#66620

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key_selector: compile with clang and -fno-exceptions - #2913

key_selector: compile with clang and -fno-exceptions#2913
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lemire commented Oct 6, 2026

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lemire commented Oct 6, 2026

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