mirror of git://sourceware.org/git/glibc.git
8a0152b61b
The current implementation relies on setting the rounding mode for different calculations (FE_TOWARDZERO) to obtain correctly rounded results. For most CPUs, this adds significant performance overhead because it requires executing a typically slow instruction (to get/set the floating-point status), necessitates flushing the pipeline, and breaks some compiler assumptions/optimizations. The original implementation adds tests to handle underflow in corner cases, whereas this implementation uses a different strategy that checks both the mantissa and the result to determine whether the result is not subject to double rounding. I tested this implementation on various targets (x86_64, i686, arm, aarch64, powerpc), including some by manually disabling the compiler instructions. Performance-wise, it shows large improvements: reciprocal-throughput master patched improvement x86_64 [1] 58.09 7.96 7.33x i686 [1] 279.41 16.97 16.46x aarch64 [2] 26.09 4.10 6.35x armhf [2] 30.25 4.20 7.18x powerpc [3] 9.46 1.46 6.45x latency master patched improvement x86_64 64.50 14.25 4.53x i686 304.39 61.04 4.99x aarch64 27.71 5.74 4.82x armhf 33.46 7.34 4.55x powerpc 10.96 2.65 4.13x Checked on x86_64-linux-gnu and i686-linux-gnu with —disable-multi-arch, and on arm-linux-gnueabihf. [1] gcc 15.2.1, Zen3 [2] gcc 15.2.1, Neoverse N1 [3] gcc 15.2.1, POWER10 Signed-off-by: Szabolcs Nagy <nsz@gcc.gnu.org> Co-authored-by: Adhemerval Zanella <adhemerval.zanella@linaro.org> Co-authored-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com> Reviewed-by: Wilco Dijkstra <Wilco.Dijkstra@arm.com> |
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| aarch64 | ||
| alpha | ||
| arc | ||
| arm | ||
| csky | ||
| generic | ||
| gnu | ||
| hppa | ||
| htl | ||
| hurd | ||
| i386 | ||
| ieee754 | ||
| loongarch | ||
| m68k | ||
| mach | ||
| microblaze | ||
| mips | ||
| nptl | ||
| or1k | ||
| posix | ||
| powerpc | ||
| pthread | ||
| riscv | ||
| s390 | ||
| sh | ||
| sparc | ||
| unix | ||
| wordsize-32 | ||
| wordsize-64 | ||
| x86 | ||
| x86_64 | ||