Backport #18532 to 2.2: Fix aarch64 build failure by removing earlyclobber#18671
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Backport #18532 to 2.2: Fix aarch64 build failure by removing earlyclobber#18671alex-moch wants to merge 1 commit into
alex-moch wants to merge 1 commit into
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The UVR macros used "+&w" (read-write + earlyclobber) as the
constraint for NEON register operands that are declared as explicit
hard-register variables via:
register unsigned char wN asm("vN") __attribute__((vector_size(16)));
The + modifier implicitly makes the operand also an input (reading the
register before the asm runs). The & (earlyclobber) modifier says "this
output may be written before all inputs are consumed." Having an
earlyclobber output on the same hard-register that is simultaneously
an input is a contradiction — GCC 16 now strictly diagnoses this.
The fix removes the & from "+&w", yielding "+w". The earlyclobber
was both incorrect (contradicts the implicit input) and unnecessary
(the physical registers are already hard-bound, so the compiler has no
freedom to assign conflicting registers anyway).
Issue openzfs#18525
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
(cherry picked from commit 3800525)
Signed-off-by: Alexander Moch <mail@alexmoch.com>
behlendorf
approved these changes
Jun 25, 2026
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The UVR macros used "+&w" (read-write + earlyclobber) as the constraint for NEON register operands that are declared as explicit hard-register variables via:
register unsigned char wN asm("vN") attribute((vector_size(16)));
The + modifier implicitly makes the operand also an input (reading the register before the asm runs). The & (earlyclobber) modifier says "this output may be written before all inputs are consumed." Having an earlyclobber output on the same hard-register that is simultaneously an input is a contradiction — GCC 16 now strictly diagnoses this.
The fix removes the & from "+&w", yielding "+w". The earlyclobber was both incorrect (contradicts the implicit input) and unnecessary (the physical registers are already hard-bound, so the compiler has no freedom to assign conflicting registers anyway).
Motivation and Context
The corresponding pull request #18532 was not backported to 2.2.10 and fails to build with GCC 16 on AArch64.
Description
Clean cherry-pick from 3800525.
How Has This Been Tested?
Tested on Gentoo with Linux 6.12.93 and GCC 16.1.0 on AArch64 with root on ZFS. No issues.
Types of changes
Checklist:
Signed-off-by.