The testcase int foo(int bar) { int i, res = 0; for (i=0; i<bar; ++i) { int *x = __builtin_alloca(4); res += *x; } return res; } uses a lot more memory than necessary because of __builtin_alloca apparently returning 16-byte aligned stack space (-O2 -fomit-frame-pointer): .L5: subl $32, %esp #, incl %edx # i leal 15(%esp), %eax #, tmp64 andl $-16, %eax #, tmp64 addl (%eax), %ecx #, res cmpl %edx, %ebx # i, bar jg .L5 #, I cannot find anything in the C standard about alloca, but alignment bigger than the requested storage size should be not needed (though this may be architecture dependent). At least playing with alignment should be moved outside of the loop, so we could save half of the wasted memory (I know the testcase is dumb, but it's at least simple). Btw. the same problem applies to 3.4, 3.3 doesn't play alignment games but still allocates 16 bytes each time (that won't be aligned to 16 bytes this way anyway?). 2.95 beats all of them again in simplicity: .L6: addl $-16,%esp addl (%esp),%eax decl %edx jnz .L6
The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid We need to keep the stack aligned sorry.
Subject: Re: alloca returning unnecessarily aligned pointer and uses too much memory pinskia at gcc dot gnu dot org wrote: > ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 ------- > The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid > > We need to keep the stack aligned sorry. Inside a function!? Or just at function callsites? Humm, the Intel compiler produces ..B1.3: # Preds ..B1.2 ..B1.4 movl $4, %eax #5.12 subl %eax, %esp #5.12 andl $-16, %esp #5.12 movl %esp, %eax #5.12 # LOE eax ebx ebp esi edi ..B1.4: # Preds ..B1.3 addl (%eax), %ebx #6.3 addl $1, %esi #4.21 cmpl %edi, %esi #4.2 jl ..B1.3 which looks like it aligns the stack after alloca, but it manages to waste less space by subtracting $4, not $32. Also if the ABI says the stack is aligned, why do we not make use of this and avoid the andl $-16, %esp -- or is the alignment only about alloca? I'm a bit confused. Richard.
Subject: Re: alloca returning unnecessarily aligned pointer and uses too much memory pinskia at gcc dot gnu dot org wrote: > ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 ------- > The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid Where is the ABI specified? Is it the "System V ABI, Intel386 Architecture Processor Supplement" document I found at http://www.caldera.com/developers/devspecs/abi386-4.pdf? This one talks about word-alignment of the stack, not 16 byte alignment. > We need to keep the stack aligned sorry.
Subject: Re: alloca returning unnecessarily aligned pointer and uses too much memory > > ------- Additional Comments From rguenth at tat dot physik dot uni-tuebingen dot de 2004-12-23 22:23 ------- > Subject: Re: alloca returning unnecessarily aligned pointer > and uses too much memory > > pinskia at gcc dot gnu dot org wrote: > > ------- Additional Comments From pinskia at gcc dot gnu dot org 2004-12-22 15:06 ------- > > The reason you cannot find anything in the C standard is because this is ABI thing so this is invalid > > Where is the ABI specified? Is it the "System V ABI, Intel386 > Architecture Processor Supplement" document I found at > http://www.caldera.com/developers/devspecs/abi386-4.pdf? > This one talks about word-alignment of the stack, not 16 byte alignment. We keep the stack 16 byte aligned as an extension to support SSE.... Honza > > > We need to keep the stack aligned sorry. > > > -- > > > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19131