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Frédéric Desbiens
ca62edd27d
#636 reported that a stack in BTCM gave a threefold tighter spread than DRAM0 for stack-heavy work. That measurement used a single code placement, which is the methodology #631 and #633 exist to correct: the cache benchmark got an alignment sweep and the interrupt handler got one, and this measurement never did. It was noticed when #637 added two threads to the same image and the figure moved -- both spreads came out near 6500 and the minima rose 15%. The recursive body is now generated at four placements and all four are measured, per placement, in one image. placement BTCM min / spread DRAM0 min / spread offset 0 41854 / 6850 42036 / 6880 offset 16 48670 / 1946 48792 / 1978 offset 32 42388 / 6914 42752 / 7018 offset 48 48914 / 1860 49198 / 6786 Reproducible across runs to within a few hundred cycles. Spread is dominated by code placement rather than by the memory holding the stack. It ranges from 1860 to 6914 depending on where the body falls in a cache line, and placement also moves the minimum by 17%, from 41854 to 49214. Against that, the memory contributes a consistent but small advantage: BTCM's minimum is lower at all four placements, by 0.4% to 0.9%. BTCM's spread beats DRAM0's decisively at one placement of the four, offset 48, at 1860 against 6786. At the other three the two are within 2% of each other. So the effect #636 reported is real where it occurs and is not a property of the part: quoting it as one invited the reader to expect it everywhere. #636's claim should be read as qualified by this. A stack in BTCM buys a small consistent improvement in the best case and a large improvement in spread at some code placements and not others. Anyone building a determinism argument on it needs the placement sweep in the loop, not a single figure. The pad nops that displace each placement execute on every recursion level rather than once, so each placement carries a slightly different constant cost, about 0.6% at the widest. That cancels in the BTCM against DRAM0 comparison, which is made at the same placement, and does not affect spread within one. Assisted-by: Claude Code (Opus 5) <noreply@anthropic.com>