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fix(rmcs_board): Avoid strict aliasing in MTIME reads #66

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Merged
qzhhhi merged 2 commits into from
Jul 6, 2026
Merged

fix(rmcs_board): Avoid strict aliasing in MTIME reads #66

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92d5e2b
fix(rmcs_board): Avoid strict aliasing in MTIME reads
qzhhhi Jul 6, 2026
25cfb98
fix tidy
qzhhhi Jul 6, 2026
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9 changes: 5 additions & 4 deletions firmware/rmcs_board/app/src/timer/timer.hpp
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Original file line number Diff line number Diff line change
Expand Up @@ -43,16 +43,17 @@ class Timer {
}

static uint32_t timestamp_quarter_us() {
// Read only the lower 32 bits of MTIME with a single 32-bit load.
// Assumes little-endian (riscv32), intentionally bypasses strict aliasing for efficiency.
return *reinterpret_cast<volatile uint32_t*>(&HPM_MCHTMR->MTIME);
// Read MTIME through its 32-bit MMIO view to avoid aliasing the SDK's uint64_t field.
const volatile uint32_t* const mtime_words =
reinterpret_cast<volatile uint32_t*>(HPM_MCHTMR_BASE);
return mtime_words[0];
}

static uint64_t timestamp64_quarter_us() {
// On riscv32, reading the 64-bit MTIME MMIO register compiles to two 32-bit loads.
// Read high-low-high and retry on rollover so the returned 64-bit timestamp is stable.
const volatile uint32_t* const mtime_words =
reinterpret_cast<volatile uint32_t*>(&HPM_MCHTMR->MTIME);
reinterpret_cast<volatile uint32_t*>(HPM_MCHTMR_BASE);
uint32_t hi1 = 0;
uint32_t lo = 0;
uint32_t hi2 = 0;
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