https://github.com/jhuber6 created https://github.com/llvm/llvm-project/pull/203809
Summary: This matches what the AMD device libraries does. We can avoid extra steps by only performing the two steps of the Newton-Raphson approximation of 1 / x. The exceptional cases should not appear in these math functions, this is local to AMDGPU, and I verified they are bitwise identical to the AMD math functions with parity in performance now. >From e08e7ee7b9705b1a4c3435099054154310a0ceb8 Mon Sep 17 00:00:00 2001 From: Joseph Huber <[email protected]> Date: Sun, 14 Jun 2026 20:56:00 -0500 Subject: [PATCH] [libclc] Use unchecked division for f64 math Summary: This matches what the AMD device libraries does. We can avoid extra steps by only performing the two steps of the Newton-Raphson approximation of 1 / x. The exceptional cases should not appear in these math functions, this is local to AMDGPU, and I verified they are bitwise identical to the AMD math functions with parity in performance now. --- libclc/clc/lib/amdgpu/math/clc_recip_fast.inc | 9 ++++++++- 1 file changed, 8 insertions(+), 1 deletion(-) diff --git a/libclc/clc/lib/amdgpu/math/clc_recip_fast.inc b/libclc/clc/lib/amdgpu/math/clc_recip_fast.inc index 9d635cc700442..e19ec82d7566d 100644 --- a/libclc/clc/lib/amdgpu/math/clc_recip_fast.inc +++ b/libclc/clc/lib/amdgpu/math/clc_recip_fast.inc @@ -6,10 +6,17 @@ // //===----------------------------------------------------------------------===// -// On AMDGPU the "fast" reciprocal is the hardware v_rcp_f32 approximation, +// On AMDGPU the "fast" reciprocal is the hardware v_rcp approximation. _CLC_OVERLOAD _CLC_DEF __CLC_GENTYPE __clc_recip_fast(__CLC_GENTYPE x) { #if defined(__CLC_SCALAR) && __CLC_FPSIZE == 32 return __builtin_amdgcn_rcpf(x); +#elif defined(__CLC_SCALAR) && __CLC_FPSIZE == 64 + // Hardware v_rcp_f64 seed refined with two Newton-Raphson iterations. This + // computes 1.0 / x without the full IEEE scaling and subnormal fixups. + __CLC_GENTYPE r = __builtin_amdgcn_rcp(x); + r = __builtin_fma(__builtin_fma(-x, r, 1.0), r, r); + r = __builtin_fma(__builtin_fma(-x, r, 1.0), r, r); + return r; #else return ((__CLC_GENTYPE)1.0) / x; #endif _______________________________________________ cfe-commits mailing list [email protected] https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
