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Explain the arm64 prefer-atomic gcov flag in the flags differential #127

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Merged
tamnd merged 1 commit into from
Oct 21, 2026
Merged

Explain the arm64 prefer-atomic gcov flag in the flags differential #127

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4 changes: 4 additions & 0 deletions config-divergences.toml
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Expand Up @@ -107,3 +107,7 @@ reason = "6.12 only asks the compiler about asm goto with outputs when GCC_ASM_G
[[divergence]]
symbol = "CC_HAS_ASM_GOTO_TIED_OUTPUT"
reason = "the tied form of the same question, asked under the same condition, see CC_HAS_ASM_GOTO_OUTPUT"

[[flags]]
flag = "-fprofile-update=prefer-atomic"
reason = "7.2 adds this to its gcov flags only when it brings in no new undefined symbol. On arm64 gcc defaults to -moutline-atomics, so the atomic counter add calls libgcc's __aarch64_ldadd8_relax and the kernel drops the flag. rucc inlines its atomics, finds nothing new and keeps the flag, and since tamnd/rucc#3772 its counters are atomic adds. On x86 both builds keep it"
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