Date: Mon, 2 Nov 2015 17:36:49 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH] configure: add gcc flags for better link-time optimization On Sun, Nov 01, 2015 at 02:56:58PM -0500, Rich Felker wrote: > On Fri, Oct 23, 2015 at 02:30:26PM +0200, Denys Vlasenko wrote: > > +# > > +# Put every function and data object into its own section: > > +# .text.funcname, .data.var, .rodata.const_struct, .bss.zerovar > > +# > > +# Previous optimization isn't working too well by itself > > +# because data objects aren't living in separate sections, > > +# they are all grouped in one .data and one .bss section per *.o file. > > +# With -ffunction/data-sections, section sorting eliminates more padding. > > +# > > +# Object files in static *.a files will also have their functions > > +# and data objects each in its own section. > > +# > > +# This enables programs statically linked with -Wl,--gc-sections > > +# to perform "section garbage collection": drop unused code and data > > +# not on per-*.o-file basis, but on per-function and per-object basis. > > +# This is a big thing: --gc-sections sometimes eliminates several percent > > +# of unreachable code and data in final executable. > > +# > > +tryflag CFLAGS_AUTO -ffunction-sections > > +tryflag CFLAGS_AUTO -fdata-sections > > + > > +# > > This is not just an optimization but going to save us from a horrible > class of compiler/assembler bugs that threatened to force dropping > support for all non-bleeding-edge toolchains: > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68178 > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66609 > https://sourceware.org/bugzilla/show_bug.cgi?id=18561 > > By putting functions/objects in their own sections, the illegal but > widespread assembler 'optimization' of resolving differences between > symbols to a constant when one or both of the symbols has a weak > definition is suppressed, simply because differences of this form are > never constants when they cross sections. > > As such I want to go ahead and apply this regardless of optimization > issues, but I think we should update the comments and commit message > to reflect that this is also working around serious toolchain issues. > I hope to get to it soon now; working on some other things at the > moment. > > BTW thanks a lot for raising the idea of using these options. If it > hadn't been for your pending patch I probably would never have thought > of this as a solution to the toolchain problems above. Unfortunately there's an issue blocking this patch: some archs' crt_arch.h asm fragments have code that assumes a "short" branch can reach _start_c/_dlstart_c. With -ffunction-sections that's not the case; the entry point and C start code can be moved arbitrarily far apart by the linker. To fix this we either need to use a fully-general branch to reach the C code, or have file-specific suppression of -ffunction-sections for crt1, dlstart, etc. I'd rather just fix the asm not to make assumptions about shortness -- some of these assumptions are dangerously close to being wrong at -O0 anyway -- but to do that I need to audit all the crt_arch.h files, find the affected ones, and fix them. I'll start taking a look and see how bad it looks. Rich
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