Fix PowerPC syscalls causing invalid code from CBE #25229
+122
−14
Conversation
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Fixes ziglang#25209. On PowerPC, some registers are both inputs to syscalls and clobbered by them. An example is r0, which initially contains the syscall number, but may be overwritten during execution of the syscall. musl and glibc use a `+` (read-write) constraint to indicate this, which isn't supported in Zig. The current implementation of PowerPC syscalls in the Zig standard library instead lists these registers as both inputs and clobbers, but this results in the C backend generating code that is invalid for at least some C compilers, like GCC, which doesn't support the specifying the same register as both an input and a clobber. This PR changes the PowerPC syscall functions to list such registers as inputs and outputs rather than inputs and clobbers. Thanks to jacobly0 who pointed out that it's possible to have multiple outputs; I had gotten the wrong idea from the documentation.
taylordotfish
added a commit
to taylordotfish/zig
that referenced
this pull request
Sep 14, 2025
As far as I can tell, this has only been a problem with the C backend (see ziglang#25229), which is specifically excluded from this inline asm call. However, it's probably best to avoid it anyway and use output registers instead. In this case, because we're ignoring r3, which is the real "output" of the syscall, we can specify r0 as the sole `->` output instead, avoiding the need for a separate variable.
taylordotfish
added a commit
to taylordotfish/zig
that referenced
this pull request
Sep 17, 2025
As far as I can tell, this has only been a problem with the C backend (see ziglang#25229), which is specifically excluded from this inline asm call. However, it's probably best to avoid it anyway and use output registers instead. In this case, because we're ignoring r3, which is the real "output" of the syscall, we can specify r0 as the sole `->` output instead, avoiding the need for a separate variable.
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
Add this suggestion to a batch that can be applied as a single commit.
This suggestion is invalid because no changes were made to the code.
Suggestions cannot be applied while the pull request is closed.
Suggestions cannot be applied while viewing a subset of changes.
Only one suggestion per line can be applied in a batch.
Add this suggestion to a batch that can be applied as a single commit.
Applying suggestions on deleted lines is not supported.
You must change the existing code in this line in order to create a valid suggestion.
Outdated suggestions cannot be applied.
This suggestion has been applied or marked resolved.
Suggestions cannot be applied from pending reviews.
Suggestions cannot be applied on multi-line comments.
Suggestions cannot be applied while the pull request is queued to merge.
Suggestion cannot be applied right now. Please check back later.
Fixes #25209.
On PowerPC, some registers are both inputs to syscalls and clobbered by them. An example is r0, which initially contains the syscall number, but may be overwritten during execution of the syscall.
musl and glibc use a
+(read-write) constraint to indicate this, which isn't supported in Zig. The current implementation of PowerPC syscalls in the Zig standard library instead lists these registers as both inputs and clobbers, but this results in the C backend generating code that is invalid for at least some C compilers, like GCC, which doesn't support the specifying the same register as both an input and a clobber:This PR changes the PowerPC syscall functions to list such registers as inputs and outputs rather than inputs and clobbers; both Clang and GCC appear to be fine with this, and the CBE-generated code looks correct. Thanks to @jacobly0 who pointed out that it's possible to have multiple outputs; I had gotten the wrong idea from the documentation.