Date: Mon, 3 Aug 2020 18:05:47 +0100 From: Jessica Clarke <jrtc27@freebsd.org> To: Dimitry Andric <dim@FreeBSD.org> Cc: src-committers <src-committers@freebsd.org>, svn-src-projects@freebsd.org, Ed Schouten <ed@nuxi.nl>, John Baldwin <jhb@freebsd.org> Subject: Re: svn commit: r363773 - projects/clang1100-import/contrib/llvm-project/compiler-rt/lib/builtins Message-ID: <F5954FB2-E301-4ADD-85AC-5CF1E4352EBF@freebsd.org> In-Reply-To: <B0A89ACC-99D6-4329-8FC9-135A953262E4@FreeBSD.org> References: <202008021807.072I7GM9059504@repo.freebsd.org> <5D6813EB-F270-412D-8C15-8A05CB6353DE@freebsd.org> <B0A89ACC-99D6-4329-8FC9-135A953262E4@FreeBSD.org>
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On 3 Aug 2020, at 17:50, Dimitry Andric <dim@FreeBSD.org> wrote: >=20 > On 2 Aug 2020, at 22:50, Jessica Clarke <jrtc27@FreeBSD.org> wrote: >>=20 >> On 2 Aug 2020, at 19:07, Dimitry Andric <dim@FreeBSD.org> wrote: >>>=20 >>> Author: dim >>> Date: Sun Aug 2 18:07:16 2020 >>> New Revision: 363773 >>> URL: https://svnweb.freebsd.org/changeset/base/363773 >>>=20 >>> Log: >>> Reapply r230021, r276851 and a few other commits to compiler-rt >>>=20 >>> Reapply r230021 (by ed): >>>=20 >>> Add a workaround to prevent endless recursion in compiler-rt. >>>=20 >>> SPARC and MIPS CPUs don't have special instructions to count >>> leading/trailing zeroes. The compiler-rt library provides fallback >>> rountines for these. The 64-bit routines, __clzdi2 and __ctzdi2, are >>> implemented as simple wrappers around the compiler built-in >>> __builtin_clz(), assuming these will expand to either 32-bit >>> CPU instructions or calls to __clzsi2 and __ctzsi2. > ... >> Are you sure this is still necessary? https://reviews.llvm.org/D42902 >> (which landed in in 2018 for 7.0.0, way after the original r230021 in >> 2012) followed by a follow-up commit for the correct SPARC macro, = fixed >> this in an OS-independent way upstream, but inside c?zdi2.c = themselves >> so you won't notice a merge conflict. >=20 > Yes, you are probably right, though the preceding comment in int_lib.h > specifically mentions: >=20 > * Instead of placing this workaround in c?zdi2.c, put it in this > * global header to prevent other C files from making the detour > * through __c?zdi2() as well. >=20 > There are indeed quite a lot of calls to __builtin_c[lt]z throughout > compiler-rt, so removing this workaround from the int_lib.h header > will possibly pessimize all of those. Is that going to work alright = for > all affected architectures, which appear to be mips64, riscv and > sparc64? I don't think it matters much. __clzdi2 is this (the preprocessed version of which you provided): dwords x; x.all =3D a; const si_int f =3D -(x.s.high =3D=3D 0); return __builtin_clz((x.s.high & ~f) | (x.s.low & f)) + (f & ((si_int)(sizeof(si_int) * CHAR_BIT))); The implementation of __clzsi2 does a bunch more work than that (not crazy, but still several times more), so I don't think you'll notice all that much. Besides, Clang is our primary compiler and won't suffer from this workaround, so I personally have no qualms about a small performance hit when using GCC and what should be a relatively cold[1] function. I'm of the view we should be as close to upstream as possible, ideally with zero diffs, so personally wouldn't carry patches like this unless it had a noticeable affect on system performance, but even then would encourage others to patch it upstream first and foremost. Jess [1] Even if compiler-rt uses it a lot, I would expect the work done by its callers, and the callers of the callers etc, to far outweigh the time taken within __clzdi2 (and similarly for other related variants). > Note also that https://bugs.llvm.org/show_bug.cgi?id=3D11663 is = *still* > open, possibly because of that reason. But it looks like interest in > the bug simply waned, and it was never closed. >=20 > In any case, I managed to get the new-new-new-mk2-style version of > mips64-xtoolchain working, and got these warnings, so indeed the fix > is now applied in two places: >=20 > /usr/src/contrib/llvm-project/compiler-rt/lib/builtins/clzdi2.c:23: = warning: "__builtin_clz" redefined > 23 | #define __builtin_clz(a) __clzsi2(a) > | > In file included from = /usr/src/contrib/llvm-project/compiler-rt/lib/builtins/clzdi2.c:13: > /usr/src/contrib/llvm-project/compiler-rt/lib/builtins/int_lib.h:112: = note: this is the location of the previous definition > 112 | #define __builtin_clz __clzsi2 > | >=20 > The preprocessed C code looks OK, in the sense that it delegates to > __clzsi2 now: >=20 > extern si_int __clzsi2(si_int); >=20 > si_int __clzdi2(di_int a) { > dwords x; > x.all =3D a; > const si_int f =3D -(x.s.high =3D=3D 0); > return __clzsi2((x.s.high & ~f) | (x.s.low & f)) + > (f & ((si_int)(sizeof(si_int) * 8))); > } >=20 > And the resulting assembly shows no further external calls: >=20 > 0000000000000000 <__clzdi2>: > 0: 67bdffe0 daddiu sp,sp,-32 > 4: ffbf0018 sd ra,24(sp) > 8: ffbc0010 sd gp,16(sp) > c: ffb00008 sd s0,8(sp) > 10: 3c1c0000 lui gp,0x0 > 14: 0399e02d daddu gp,gp,t9 > 18: 679c0000 daddiu gp,gp,0 > 1c: 0004103f dsra32 v0,a0,0x0 > 20: 2c430001 sltiu v1,v0,1 > 24: 0003802f dnegu s0,v1 > 28: 2463ffff addiu v1,v1,-1 > 2c: 00621824 and v1,v1,v0 > 30: 00042000 sll a0,a0,0x0 > 34: 00902024 and a0,a0,s0 > 38: df990000 ld t9,0(gp) > 3c: 0320f809 jalr t9 > 40: 00642025 or a0,v1,a0 > 44: 32100020 andi s0,s0,0x20 > 48: 02021021 addu v0,s0,v0 > 4c: dfbf0018 ld ra,24(sp) > 50: dfbc0010 ld gp,16(sp) > 54: dfb00008 ld s0,8(sp) > 58: 03e00008 jr ra > 5c: 67bd0020 daddiu sp,sp,32 >=20 > In summary, I'm fine with dropping this part of r230021, as long as > everybody is convinced this does not pessimize the rest too much. >=20 > -Dimitry
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