On Wed, Oct 20, 2021 at 4:18 AM Richard Biener <richard.guent...@gmail.com> wrote: > > On Wed, Oct 20, 2021 at 12:40 PM Xu Dianhong <dianho...@gmail.com> wrote: > > > > Many thanks for your explanation. I got the meaning of operands. > > The "addpd b(%rip), %xmm0" instruction needs "b(%rip)" aligned otherwise it > > will rise a "Real-Address Mode Exceptions". > > I haven't considered this situation "b(%rip)" has an address dependence of > > "a(%rip)" before. I think this situation could be resolved on the assembler > > side except for this dummy code like "movapd 0x200b37(%rip),%xmm1, ... > > addpd 0x200b37(%rip),%xmm0 ". > > Of course the compiler will only emit instructions which have the > constraint of aligned memory > when the memory is known to be aligned. That's why I wonder why you > would need such > option. "Real-Address Mode Exceptions" may point to the issue, but I > wonder what's different > in real mode vs. protected mode - even with segmentation the alignment > of objects should > prevail unless you play linker"tricks" that make global objects have > different alignment - but > then it's better to adjust the respective hooks to not falsely claim > such alignment. Consider > for example > > if ((uintptr_t)&a & 0x7) > foo(); > else > bar(); > > GCC will optimize the branch statically to always call foo if 'a' > appears to be aligned, > even if you later try to "override" this with an option. Alignment is > not only about > moves, it's also about knowledge about low bits in addresses and about > alias analysis where alignment constrains how two objects can overlap. > > So - do not lie to the compiler! A late "workaround" avoiding aligned > SSE moves isn't a proper fix. >
The motivations are 1. AVX non-load/store ops work on unaligned memory. Unaligned load/store on aligned memory is as fast as aligned load/store on Intel AVX machines. The new switch makes load/store consistent with other AVX ops. 2. We don't properly align the stack for AVX on Windows. This can be used as a workaround for -mavx on Windows. We can change TARGET_USE_UNALIGNED_VECTOR_MOVE to require AVX. -- H.J.
