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CPU features are not passed to clang when assembling #10411

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CPU features are not passed to clang when assembling#10411
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enhancementSolving this issue will likely involve adding new logic or components to the codebase.upstreamAn issue with a third party project that Zig uses.zig ccZig as a drop-in C compiler feature
Milestone
0.16.0

Description

@silversquirl
silversquirl
opened on Dec 26, 2021

Zig Version

0.10.0-dev.40+303bad998

Steps to Reproduce

Create an ARM assembly file named test.s with the following contents:

mov r1, #0

Compile that file to an object using zig build-obj --verbose-cc -target arm-freestanding -mcpu cortex_m0plus+thumb2 test.s

Expected Behavior

Zig passes the +thumb2 option to clang, which correctly assembles the thumb2 instruction (though this particular CPU feature won't work until llvm/llvm-project#52878 is fixed).

Actual Behavior

The +thumb2 feature is not passed to zig clang:

$ zig build-obj --verbose-cc -target arm-freestanding -mcpu cortex_m0plus+thumb2 test.s
/home/silver/.opt/zig/zig clang -fno-caret-diagnostics -target arm-unknown-unknown-eabi -mcpu=cortex-m0plus -ffreestanding -c -o /home/silver/.cache/zig/tmp/c60723f3c667529f-test.o test.s
error(compilation): clang failed with stderr: zig: warning: argument unused during compilation: '-fno-caret-diagnostics' [-Wunused-command-line-argument]
zig: warning: argument unused during compilation: '-ffreestanding' [-Wunused-command-line-argument]
test.s:1:1: error: invalid instruction, any one of the following would fix this:
mov r1, #0
^
test.s:1:9: note: operand must be a register in range [r0, r15]
mov r1, #0
        ^
test.s:1:1: note: instruction requires: thumb2
mov r1, #0
^

test.s:1:1: error: unable to build C object: clang exited with code 1

Activity

  1. silversquirl
    added
    bugObserved behavior contradicts documented or intended behavior
    on Dec 26, 2021
  2. silversquirl
    changed the title [-]CPU flags are not passed to clang[/-] [+]CPU features are not passed to clang[/+] on Dec 26, 2021

  3. akovaski commented on Dec 27, 2021

    @akovaski
    akovaski
    on Dec 27, 2021

    Just some info:
    This is already somewhat known (Not that there's a duplicate issue on github), see the .assembly part of addCCArgs in Compilation.zig:

    // The Clang assembler does not accept the list of CPU features like the
// compiler frontend does. Therefore we must hard-code the -m flags for
// all CPU features here.

    I'm not sure what exactly this comment is referring to, but I suspect it is due to clang's -Xassembler argument not passing -target-feature to the assembler [1] [2]. (Zig uses -Xclang -target-feature -Xclang -<feature> to pass target features for C files.)

    So manually adding exceptions is the current state. I expect Zig could pass -target-feature if calling the Clang assembly compiler directly with the -cc1as flag (.e.g clang-13 -cc1as -triple riscv64 ./os/entry.s -target-feature -relax). You'd also have to adjust how Zig passes all the other flags to Clang for assembly files, because clang -cc1as has different flags from clang.

    (P.S. There's also a corresponding -cc1 flag for Clang's C compiler)

  4. andrewrk
    added
    enhancementSolving this issue will likely involve adding new logic or components to the codebase.
    zig ccZig as a drop-in C compiler feature
    arch-arm32-bit Arm
    on Dec 27, 2021
  5. andrewrk
    added this to the 0.10.0 milestone on Dec 27, 2021
  6. andrewrk
    added
    contributor friendlyThis issue is limited in scope and/or knowledge of Zig internals.
    on Dec 27, 2021

  7. zheng commented on Jan 4, 2022

    @zheng
    zheng
    on Jan 4, 2022

    Just some info: This is already somewhat known (Not that there's a duplicate issue on github), see the .assembly part of addCCArgs in Compilation.zig:

    // The Clang assembler does not accept the list of CPU features like the
// compiler frontend does. Therefore we must hard-code the -m flags for
// all CPU features here.

    I'm not sure what exactly this comment is referring to, but I suspect it is due to clang's -Xassembler argument not passing -target-feature to the assembler [1] [2]. (Zig uses -Xclang -target-feature -Xclang -<feature> to pass target features for C files.)

    So manually adding exceptions is the current state. I expect Zig could pass -target-feature if calling the Clang assembly compiler directly with the -cc1as flag (.e.g clang-13 -cc1as -triple riscv64 ./os/entry.s -target-feature -relax). You'd also have to adjust how Zig passes all the other flags to Clang for assembly files, because clang -cc1as has different flags from clang.

    (P.S. There's also a corresponding -cc1 flag for Clang's C compiler)

    Very helpful - thanks for all the context!

    I was able to run the clang compiler frontend and pass in the target feature in the example in the format you described. I'm going to try and get the various assembly flags right and put up a PR.

  8. andrewrk
    modified the milestones: 0.10.0, 0.11.0 on Sep 14, 2022

  9. mikdusan commented on Jan 26, 2023

    @mikdusan
    mikdusan
    on Jan 26, 2023
    Member

    found this stalled patch from '2014, though we'd need a few more passthru-args enabled: https://reviews.llvm.org/D6170

  10. andrewrk
    modified the milestones: 0.11.0, 0.11.1 on Jul 20, 2023

  11. richarddd commented on Jan 19, 2024

    @richarddd
    richarddd
    on Jan 19, 2024

    Can we expect this in 0.11.1 or 0.12? :)

  12. 26 remaining items

  13. alexrp
    added this to the urgent milestone on Aug 1, 2025
  14. alexrp
    added 2 commits that reference this issue on Aug 3, 2025

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    313a29c

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    1bcfaed
  15. alexrp
    added a commit that references this issue on Aug 13, 2025

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    cb4ed58
  16. alexrp
    added 7 commits that reference this issue on Aug 25, 2025

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    60c5f6a

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    73ba0c9

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    768ed41

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    b81c559

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    5375a3e

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    df245c3

    zig cc: pass CPU features to Clang using the new -Xclangas when assem…

    8c47dda
  17. alexrp
    closed this as completedin 8c47ddaon Aug 30, 2025
  18. alexrp
    removed their assignment
    on Sep 2, 2025
  19. andrewrk
    modified the milestones: urgent, 0.16.0 on Mar 17, 2026
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