On 12/21/2010 06:12 PM, Richard Earnshaw wrote:
On Tue, 2010-12-21 at 12:12 +0800, Jie Zhang wrote:
Hi,
While working on a bug, I found some code in ARM port that I don't
understand.
In ARM_LEGITIMIZE_RELOAD_ADDRESS and arm_legitimize_address, we allow a
very small offset for DImode addressing.
In ARM_LEGITIMIZE_RELOAD_ADDRESS:
if (MODE == DImode || (MODE == DFmode&& TARGET_SOFT_FLOAT)) \
low = ((val& 0xf) ^ 0x8) - 0x8; \
In arm_legitimize_address
/* VFP addressing modes actually allow greater offsets, but for
now we just stick with the lowest common denominator. */
if (mode == DImode
|| ((TARGET_SOFT_FLOAT || TARGET_VFP)&& mode == DFmode))
{
low_n = n& 0x0f;
n&= ~0x0f;
if (low_n> 4)
{
n += 16;
low_n -= 16;
}
}
AFAIK, we could use two LDRs, or one LDRD, or one VLDR to access DImode
in memory when the address is in the form of (REG + CONST_INT). The
offset ranges for these three cases are:
LDR -4095,4091
LDRD -255,255
VLDR -1020,1020&& (ADDR& 3) == 0
The original code was designed to exploit LDM(IA,IB,DB,DA) which would
have the offset ranges described. On earlier ARM chips (certainly up to
and including ARM7TDMI) it was a significant win to do it that way (add
a constant to the address register and then use LDM was faster than two
LDR instructions).
That's no-longer true on modern chips, LDM is often slower than
individual LDR insns now.
Thanks! Now I see. So I think the original code is still needed but
should be used only for such earlier ARM chips. I will send the updated
patch to gcc-patches mailing list.
--
Jie Zhang
