Date: Sun, 1 Nov 2015 14:56:58 -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 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. Rich
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