From: Nick Desaulniers > Sent: 18 August 2020 21:59 > > On Tue, Aug 18, 2020 at 1:27 PM Nick Desaulniers > <ndesaulniers@xxxxxxxxxx> wrote: ... > > > -ffreestanding as it stands today does have __builtin_memcpy and > > > friends. But you need to then use #define memcpy __builtin_memcpy etc, > > > which is messy and also doesn't fully express what you want. #pragma, or > > > even just allowing -fbuiltin-foo options would be useful. > > I do really like the idea of -fbuiltin-foo. For example, you'd specify: > > -ffreestanding -fbuiltin-bcmp > > as an example. `-ffreestanding` would opt you out of ALL libcall > optimizations, `-fbuiltin-bcmp` would then opt you back in to > transforms that produce bcmp. That way you're informing the compiler > more precisely about the environment you'd be targeting. It feels > symmetric to existing `-fno-` flags (clang makes -f vs -fno- pretty > easy when there is such symmetry). And it's already convention that > if you specify multiple conflicting compiler flags, then the latter > one specified "wins." In that sense, turning back on specific > libcalls after disabling the rest looks more ergonomic to me. > > Maybe Eli or David have thoughts on why that may or may not be as > ergonomic or possible to implement as I imagine? You might want -fbuiltin-bcmp=my_bcmp_function so that you can specify the actual elf symbol name to be used. I was recently trying to compile an application so that it would run on a system with an old libc. Avoiding explicit calls to new functions wasn't a problem, but I couldn't do anything about the memcpy() calls generated by gcc for structure copies. Due to the silly glibc fiasco with memcpy going backwards I'd either need to force out a reference to the old version of memcpy or a reference to memove - but neither is possible. I then tried a C++ program and gave up because 'char traits' was all different. I then realised I need to remove all references to std::string to get any kind of efficient object code :-) David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)
