Date: Thu, 8 Nov 2012 16:11:36 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Jung-uk Kim <jkim@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, src-committers@FreeBSD.org, Dimitry Andric <dim@FreeBSD.org> Subject: Re: svn commit: r242715 - head/sys/conf Message-ID: <20121108151338.G2278@besplex.bde.org> In-Reply-To: <509AE28E.4020908@FreeBSD.org> References: <201211072215.qA7MFSYX017265@svn.freebsd.org> <509AE28E.4020908@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
On Wed, 7 Nov 2012, Jung-uk Kim wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 2012-11-07 17:15:28 -0500, Dimitry Andric wrote: >> Author: dim Date: Wed Nov 7 22:15:28 2012 New Revision: 242715 >> URL: http://svnweb.freebsd.org/changeset/base/242715 >> >> Log: For kernel builds with PROFLEVEL >= 1, such as LINT, don't >> attempt to use the -falign-functions option if the compiler is >> clang, as the flag is not supported. This just breaks the warning. The alignment is needed for PROFLEVEL >= 1 (that is any profiling at all) to actually work. Constants in <machine/profile.h> depend on it, or rather the reverse -- the alignment supports these constants. PROFLEVEL >= 2 depends more critically on the alignment, but is more completely broken. >> Modified: head/sys/conf/kern.pre.mk >> ============================================================================== >> > - --- head/sys/conf/kern.pre.mk Wed Nov 7 22:11:38 2012 (r242714) >> +++ head/sys/conf/kern.pre.mk Wed Nov 7 22:15:28 2012 (r242715) @@ >> -102,7 +102,10 @@ CLANG_NO_IAS= -no-integrated-as .endif >> >> .if defined(PROFLEVEL) && ${PROFLEVEL} >= 1 -CFLAGS+= -DGPROF >> -falign-functions=16 +CFLAGS+= -DGPROF +.if ${COMPILER_TYPE} != >> "clang" +CFLAGS+= -falign-functions=16 +.endif .if ${PROFLEVEL} >= >> 2 CFLAGS+= -DGPROF4 -DGUPROF PROF= -pg -mprofiler-epilogue > > FYI, PROF needs a similar work-around: > > % clang -c -pg -mprofiler-epilogue test.c > clang: warning: argument unused during compilation: '-mprofiler-epilogue' Again, this just breaks the warning. The main part of the complete brokenness of PROFLEVEL >= 2 is that gcc doesn't ignore this option, but it just doesn't work. I got tired of fixing breakages of this a few rounds ago. A previous round used -finstrument-functions, but this generated bloated code and is much worse now since it now generates instrumentation for all the little inline functions like curthread that you don't want to instrument, unless you use large complicatins to kill their instrumentation selectively (there is an attribute for this, but there seems to be no compiler flag for a less-selective kill). clang's lack of support for other gcc flags is annoying. One such flag is -mpreferred-stack-boundary. This is important for reducing kernel stack bloat with gcc. clang has better stack alignment so it doesn't really need the flag, but using it gives anyoying warnings so you have to ifdef for it... clang also has lots of floating point bugs. Some are shared with gcc but act a little differently so that the workarounds for them in gcc don't ameliorate them. The last 3 that I noticed are: - overflowing multiplications of constants are misoptimized to infinity with no overflow. gcc has a similar bug for underflowing multiplications of constants. clang has this too. libm has many lines of work-arounds for the missing underflow but not for the missing overflow. Perhaps the missing exceptions are permitted by the default for the FENV_ACCESS pragma, and in that case the bug might be a different default. Neither gcc nor clang supports C99 here, so they have essentially null support for the pragma and null support for what it controls. gcc defaults to a setting that is fairly fail-safe, and even documents this in its info page. - clang fails to optimize division by (integer) 2 into multiplication by (floating point with the correct type) 0.5 in the case of long doubles on one supported arch (i386 IIRC). Both compilers optimize all other cases, including floats and doubles on all arches. clang with -ffast-math optimizes the broken case, but -ffast-math is generally unsafe and not even fast; it shouldn't be necessary for this, since this is one of the few floating point strength reductions that is always safe. This detail is not documented for either clang or gcc. clang doesn't even document the existence of -ffast-math in its man page. Several uncommitted libm functions use the spelling of (integer) 2 to avoid ifdefs and/or different code to get the type right for 0.5. The result is that the case of long doubles on i386 is pessimized. clang has many other efficiency and ABI problems on i386, including the next one. - clang doesn't align long doubles on i386 when this is possible. The i386 ABI is unfortunately broken for long doubles (and doubles), and only requires 4-byte alignment. However, 8-byte alignment is possible in many cases (e.g., for all auto and static objects). Both compilers use 8-byte alignment for doubles in all cases and should do the same for long doubles (or perhaps 16-byte, but that just waste space on most or all current CPUs AFAIR, even for ABI where the ABI requires at least the size of a long double to be 16). clang only does 4-byte alignment for long doubles, at least in the important case of static constants. (static constants are handled by both compilers by replacing them by unnamed objects having better locality and alignment if possible. x86 allows just a few cases where the object is an immediate constant.) I mostly don't notice other floating point bugs in clang because I noticed them before and have scattered work-arounds for them. Not very many. The largest ones involve FLT_EVAL_METHOD, float_t and double_t. On i386, these are broken in both the clang and the FreeBSD headers. The headers just use the gcc values with no ifdefs. Correct ifdefs are not easy to write because compiler pre-defines like __FLT_EVAL_METHOD are even more broken. The broken FLT_EVAL_METHOD, float_t and double_t mainly give inefficencies because they are mostly fail-safe. When you try to fix them, you get larger ABI problems. Bruce
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20121108151338.G2278>