https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94092
Jeffrey A. Law <law at gcc dot gnu.org> changed:
What |Removed |Added
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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.
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;
}
