[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #12 from Florian Weimer fweimer at redhat dot com --- (In reply to Chip Salzenberg from comment #11) Indeed, 16 is required by the ABI; see http://www.x86-64.org/documentation/abi.pdf page 12. Only the SIMD __m256 is bigger than 16, and there seems no end to Intel's extensions to SIMD registers, so holding at 16 seems like the Right Thing. Even for allocations smaller than 16 bytes?
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #11 from Chip Salzenberg chip at pobox dot com --- Indeed, 16 is required by the ABI; see http://www.x86-64.org/documentation/abi.pdf page 12. Only the SIMD __m256 is bigger than 16, and there seems no end to Intel's extensions to SIMD registers, so holding at 16 seems like the Right Thing.
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 Florian Weimer fweimer at redhat dot com changed: What|Removed |Added CC||fweimer at redhat dot com --- Comment #9 from Florian Weimer fweimer at redhat dot com --- On x86-64, Both jemalloc and tcmalloc return 8 byte aligned pointers for sizes of 8 and less. It's not true that malloc guarantuees 16-byte alignment there. You cannot look at glibc malloc implementation details and infer ABI properties from that.
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 Jakub Jelinek jakub at gcc dot gnu.org changed: What|Removed |Added CC||jakub at gcc dot gnu.org --- Comment #10 from Jakub Jelinek jakub at gcc dot gnu.org --- Then jemalloc and tcmalloc are just broken on x86_64.
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #8 from Chip Salzenberg chip at pobox dot com --- Further research says that the alignment of a malloc(N) will be = N if there is a basic type that requires alignment N. So we may be able to ramp this up quite a bit.
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #7 from Chip Salzenberg chip at pobox dot com --- Should this ticket have status CONFIRMED ? Also I suspect it's been fixed in trunk...
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #6 from Chip Salzenberg chip at pobox dot com 2013-03-29 06:05:19 UTC --- May I have this accepted?
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 Richard Biener rguenth at gcc dot gnu.org changed: What|Removed |Added Keywords||missed-optimization Target||x86_64-*-*, i?86-*-* Status|UNCONFIRMED |NEW Last reconfirmed||2013-03-26 Ever Confirmed|0 |1 --- Comment #5 from Richard Biener rguenth at gcc dot gnu.org 2013-03-26 14:27:03 UTC --- Confirmed.
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 H.J. Lu hjl.tools at gmail dot com changed: What|Removed |Added CC||hjl.tools at gmail dot com --- Comment #1 from H.J. Lu hjl.tools at gmail dot com 2013-03-25 20:04:44 UTC --- (In reply to comment #0) Observed malloc alignment for the i386 ABI is double POINTER_SIZE. BITS_PER_WORD, the current default, is usually too small. (It's right only on X32.) Proposed patch: --- gcc/config/i386/i386.h (revision 197055) +++ gcc/config/i386/i386.h (working copy) @@ -815,6 +815,14 @@ x86_field_alignment (FIELD, COMPUTED) #endif +/* The maximum alignment 'malloc' honors. + + This value is taken from glibc documentation for memalign(). It may + be up to double the very conservative GCC default. This should be safe, + since even the GCC 4.8 default of BIGGEST_ALIGNMENT usually worked. */ + +#define MALLOC_ABI_ALIGNMENT (POINTER_SIZE * 2) + This isn't correct for x32, whose pointer size is 4 bytes with malloc aligned to 16 bytes.
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #2 from Chip Salzenberg chip at pobox dot com 2013-03-25 21:35:19 UTC --- I'm a bit skeptical of that. Glibc malloc alignment is 2 * sizeof(void*), and void* in X32 is 32 bits. Unless X32 code uses the x86_64 libc, I am confused. PS: Hi, HJ
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #3 from H.J. Lu hjl.tools at gmail dot com 2013-03-25 22:07:18 UTC --- (In reply to comment #2) I'm a bit skeptical of that. Glibc malloc alignment is 2 * sizeof(void*), and void* in X32 is 32 bits. Unless X32 code uses the x86_64 libc, I am confused. PS: Hi, HJ Hi Chip, The current glibc has #ifndef MALLOC_ALIGNMENT # if !SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_16) /* This is the correct definition when there is no past ABI to constrain it. Among configurations with a past ABI constraint, it differs from 2*SIZE_SZ only on powerpc32. For the time being, changing this is causing more compatibility problems due to malloc_get_state and malloc_set_state than will returning blocks not adequately aligned for long double objects under -mlong-double-128. */ # define MALLOC_ALIGNMENT (2 * SIZE_SZ __alignof__ (long double) \ ? __alignof__ (long double) : 2 * SIZE_SZ) # else # define MALLOC_ALIGNMENT (2 * SIZE_SZ) # endif #endif
[Bug target/56726] i386: MALLOC_ABI_ALIGNMENT is too small (usually)
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56726 --- Comment #4 from Chip Salzenberg chip at pobox dot com 2013-03-25 22:35:57 UTC --- If I'm reading that correctly, it seems to agree with my patch. It looks like MALLOC_ABI_ALIGNMENT of POINTER_SIZE*2 is always either correct or smaller than necessary, but never too large. If MALLOC_ABI_ALIGNMENT is smaller than necessary then optimizations may be missed (depending on the values). But if it is too large then performance *will* suffer. It might even cause exceptions from unaligned accesses, but i386 is very forgiving, so it'll just be slower for no apparent reason. Perhaps the glibc version differences in malloc should be advertised with __attribute__ on the malloc declarations. Perhaps a new pragma or attribute is required to do this 100% right. But in the meantime I like POINTER_SIZE*2.
