[Patch,AVR]: Fix PR39386 (x << x and x >> x)

[Georg-Johann Lay]

This is a fix for pathological, variable shift offset shifts of
the form x << x resp. x >> x.

Such shifts need a shift register which might overlap with the
shift operand.

unsigned char shift (unsigned int x)
{
    return x << x;
}


Without patch, note r24 is part of operand and used in loop:

shift:
	rjmp 2f
1:	lsl r24
	rol r25
2:	dec r24
	brpl 1b
	ret

With patch use tmp_reg (R0) as counter:

shift:
	mov r0,r24
	rjmp 2f
1:	lsl r24
	rol r25
2:	dec r0
	brpl 1b
	ret

Patch as obvious. Increased instruction length is already
taken into account because the RO = Rx will be needed if
Rx is used afterwards, anyway.

Ok to install?

Johann


	PR target/39386
	* config/avr/avr.c (out_shift_with_cnt): Use tmp_reg as
	shift counter for x << x and x >> x shifts.

-------------- next part --------------
A non-text attachment was scrubbed...
Name: pr39386.diff
Type: text/x-patch
Size: 567 bytes
Desc: not available
URL: <http://gcc.gnu.org/pipermail/gcc-patches/attachments/20110725/5c9ff71e/attachment.bin>