https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94092
--- Comment #15 from rguenther at suse dot de <rguenther at suse dot de> --- On Fri, 30 Apr 2021, law at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94092 > > Jeffrey A. Law <law at gcc dot gnu.org> changed: > > What |Removed |Added > ---------------------------------------------------------------------------- > CC| |law at gcc dot gnu.org > > --- Comment #14 from Jeffrey A. Law <law at gcc dot gnu.org> --- > WRT Jim's comment about alignments in c#6. > > Right now a pointer's alignment is really only used to eliminate unnecessary > masking -- we don't propagate a pointer's known alignment to improve the known > alignment of memory operations involving that pointer. Hmm, but we do - in set_mem_attributes_minus_bitpos. But maybe below is for way later RTL generation, > This is something I'd cobbled together for a closely related issue which will > try to increase the known alignment of a MEM by using the alignment of a > pointer to that MEM. > > We've gone a slightly different (and more effective) route for that internal > issue, but this may still be worth polishing a bit and submitting. > > diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c > index 972512e81..be9ff76b5 100644 > --- a/gcc/emit-rtl.c > +++ b/gcc/emit-rtl.c > @@ -859,6 +859,28 @@ gen_rtx_MEM (machine_mode mode, rtx addr) > we clear it here. */ > MEM_ATTRS (rt) = 0; > > + /* If we can deduce a higher alignment for the memory access > + based on the pointer, then it's advantageous to do so. */ > + unsigned int align = 0; > + if (REG_P (addr) > + && REG_POINTER (addr)) > + align = REGNO_POINTER_ALIGN (REGNO (addr)); > + else if (GET_CODE (addr) == PLUS > + && REG_P (XEXP (addr, 0)) > + && REG_POINTER (XEXP (addr, 0)) > + && REGNO_POINTER_ALIGN (REGNO (XEXP (addr, 0))) > + && GET_CODE (XEXP (addr, 1)) == CONST_INT) > + { > + unsigned int tmp = 1 << (ffs_hwi (INTVAL (XEXP (addr, 1))) - 1); > + /* ALIGN is in bits. */ > + tmp <<= 3; > + align = REGNO_POINTER_ALIGN (REGNO (XEXP (addr, 0))); > + align = (align > tmp) ? tmp : align; > + } > + > + if (align > mode_mem_attrs[(int) mode]->align) > + set_mem_align (rt, align); > + > return rt; > } > >
