On Thu, Aug 20, 2020 at 04:33:03PM -0700, Linus Torvalds wrote: > On Thu, Aug 20, 2020 at 10:56 AM Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote: > > > > Clang's interpretation is more useful for embedded, since you can use > > -fno-builtin-foo and avoid calling __builtin_foo directly, and be > > guaranteed that there will be no calls to foo that you didn't write > > explicitly (outside of memcpy/memset/memcmp). In this case you are free > > to implement foo with non-standard semantics, or avoid implementing it > > altogether, and be reasonably confident that it will all work. > > Honestly, I think concentrating on whether __builtin_foo() works or > not misses the point entirely. > > That has never _ever_ been a problem for us, and I doubt it has been a > problem for anybody else either. > > If you use __builtin_memcpy() in your source tree, then why would you > possibly ever want to disable it? And if you _don't_ use it, then > again - why would you ever want to disable it? > > No, the real (and only) problem has always been about the compilers > magically and silently "recognizing" certain source patterns, and > turning them into function calls behind the users back. > > And that has nothing to do with __builtin_foo(). At least it > _shouldn't_ have anything to do with it. > There seems to be some confusion here. The recognition and __builtin_foo() go hand-in-hand: memcpy() is special _because_ the compiler defines it to be __builtin_memcpy(); and the compiler turns the patterns into __builtin_foo() calls, which just end up as a call to foo() if they can't be inlined. The no-builtin- options _don't_ disable __builtin_ functions. They remove the default definition of foo() as __builtin_foo(). Take the problem that instigated this thread. __builtin_stpcpy() doesn't work in the kernel because the fallback, stpcpy(), isn't implemented. The optimization is doing: sprintf(buf,"%s",s) -> __builtin_sprintf(buf,"%s",s) -> __builtin_stpcpy(buf,s)-buf -> stpcpy(buf,s)-buf Now, further below, you basically say this is an example of the compiler taking something non-stpcpy() and turning it into stpcpy(), and you ask for a no-magic-stpcpy that would stop this optimization. That's what clang's no-builtin-stpcpy already does. The only extra thing it does is that the compiler will also not touch an explicit call to stpcpy(), but you can still call __builtin_stpcpy() if you really want it. This is what was going on in that LZ4 memcpy() issue: the compiler was faithfully compiling code like memcpy(d,s,8) into a call to memcpy() because we told it not to define memcpy() as __builtin_memcpy(), by compiling for a freestanding environment. This is why I'm saying clang's no-builtin-foo option is useful for embedded: it doesn't prevent the programmer using __builtin_foo(), it prevents the _compiler_ using __builtin_foo() on its own. > So this is things like the compiler silently seeing "oh, you called > your function 'free()', so we know that the stores you did to it are > dead and we'll remove them". > > Or this is the compiler doing "Oh, you did four stores of zero in a > row, and and you asked for size optimizations, so we'll turn those > into a 'bzero()' call". This one is slightly different from the previous one. The first case is really a call to __builtin_free(). This one is turning something that wasn't a function call into __builtin_bzero(), and I would hope that no-builtin-bzero would stop it as well. OTOH, the compiler is free to turn it into memset(), just like it could for structure/array initializers. The memcpy/memset/memcmp family is a bit of an edge case: the compiler requires them to be defined even for freestanding environments, so you can't in general stop the compiler from turning something into memset(). (That -ffreestanding stops gcc from turning loops into memset() is a pragmatic recognition that some people are going to try to implement memset() in C.) > > Or the compiler doing completely broken things, and turning a > "!memcmp()" expression into a "!bcmp()" because the compilier > incorrectly assumes it's faster. Stop it with the bcmp-shaming already. bcmp _can_ be implemented faster than memcmp, and it's no great loss if it isn't, since then it'll just be an alias to memcmp in any sensible libc. > > Notice? Not a single one of those had any __builtin_xyz() pattern in > them. Quite the reverse. The compiler took something completely > different, and assumed builtin semantics without us having told it to. > > So I think "-fno-builtin-xyz" is barking *completely* up the wrong > tree. It's missing the problem. The problem is not "I have some > builtin patterns, here, you can use them". Nope: in a hosted environment, xyz() _is_ __builtin_xyz(), and that is almost always a good thing for 99% of the code out there: you tell it to use builtin semantics by choosing to compile for a hosted environment. If you want something in between freestanding and hosted, you absolutely need some way to tell the compiler exactly which xyz()'s can be treated as __builtin_xyz() and which ones shouldn't. The no-builtin- flags allow you to start from a hosted environment and turn off the specialness of the functions that you don't want to be special. The proposed builtin- flags would allow you to start from freestanding and turn on the specialness of the functions that you do want to be special. > > It's the same as all the vector intrinsics. Those don't hurt anybody - > as long as they only get used when people use the intrinsics. If the > compiler starts to suddenly use vector intrinsics without being told > to, *THAT* can be a problem. But there is never any reson to turn off > any particular intrinsic otherwise. > > If you don't want it used, don't use it. And if you do use it, the > compiler generates the vector code sequence. It's that simple. > > So to me, a compiler flag like "-fno-builtin-memcpy" is completely > insane. The flag adds absolutely no value. > > The real value would be "-fno-magic-bcmp" which turns off stupid > optimizations that magically turn non-bcmp things into bcmp. But it > should not turn off *actual* __builtin_bcmp() if such a thing exists > and people want to explicitly use it. > > Linus
