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Jan ŠléglandClaude Fable 5.1
580ac08478
The driver's Log2 has an absolute error that does not shrink with the result, so near 1.0 the relative error explodes: on an RTX 3080, v_log_f32 of 1 - 2^-24 is 23,947,836 ULP off. Measured on RDNA2 (gfx1036), the hardware's log2 is within 1 ULP of the correctly rounded value, and an eight-term series in x - 1 is within 2 ULP of both for |x - 1| <= 1/8. The lowering keeps the driver's Log2 and takes the series only inside that band when the two differ by more than 4 ULP, so RADV's native result stays bit-exact and NVIDIA's is within 6 ULP inside the band. agc_driver_log_near_one pins 64 inputs per type from the oracle with per-lane tolerances. Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_012GLfKH7Sqe51wX254k1XsV