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Docs: the rc ray tracer difference under stack promotion is heap layout - #608
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Two throwaway heap allocations at the point of the two promoted objects bring the promoted build back to the unpromoted time. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Summary
#605 documented the ray tracer as about 8% slower under `-mm=rc` with stack promotion, and called the cause not understood. It is heap layout. The two promoted objects were the program's first heap allocations, so without them every later object lands elsewhere. Put two throwaway heap allocations of the same size back at the same point and the promoted build runs at the unpromoted speed:
The hot functions' IR is identical in every build. Docs only.
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