In the ARM architecture: Under ARM mode, the PC(r15) register value is
"address of current insn + 8", while in Thumb mode (including Thumb-2),
it is "address of current insn + 4"
With the way the cbz/cbnz immediate is defined, which is 0--126 (even),
the branch range is then: +4 to +130 :)
On the issue of the doloop_end pattern for ARM/Thumb-2 in the context of
enabling SMS, I actually have tried it in the past. IIRC, I used a
"subs+bne" assembly sequence then.
Chung-Lin
On 2010/11/29 下午 04:37, Revital1 Eres wrote:
OK, thanks for your answer.
Is there other instruction for backward jumps that can be used
to replace cmp to zero+bne?
btw, in the following link of thumb-2 instructions set I understood that
the label can appear before the branch so it was confusing...
CB{N}Z Rn,<label> If Rn {== or !=} 0 then PC := label. label is (this
instruction + 4-130).
http://infocenter.arm.com/help/topic/com.arm.doc.qrc0001l/QRC0001_UAL.pdf
Thanks,
Revital
From: Chung-Lin Tang<clt...@codesourcery.com>
To: Revital1 Eres/Haifa/i...@ibmil
Cc: linaro-toolchain@lists.linaro.org
Date: 29/11/2010 10:21 AM
Subject: Re: A question about thumb2 cbnz/cbz implementation in
thumb2.md
Unfortunately, cbnz/cbz has a 6-bit:'0' (7-bit aligned to 2) immediate
offset that is *zero extended*, i.e. it is only for forward branches.
Chung-Lin
On 2010/11/29 下午 04:14, Revital1 Eres wrote:
Hello,
I have a question about cbnz/cbz thumb-2 instruction implementation in
thumb2.md file:
I have an example where we jump to a label which appears before the
branch;
for example:
L4
...
cmp r3, 0
bne .L4
It seems that cbnz instruction should be applied in this loop; replacing
cmp+bne; however, cbnz fails to be applied as diff = ADDRESS (L4) -
ADDRESS
(bne .L4) is negative and according to thumb2_cbz in thumb2.md it should
be 2<=diff<=128 (please see snippet below taken from thumb2_cbz).
So I want to double check if the current implementation of thumb2_cbnz
in thumb2.md needs to be changed to enable it.
The following is from thumb2_cbnz in thumb2.md:
[(set (attr "length")
(if_then_else
(and (ge (minus (match_dup 1) (pc)) (const_int 2))
(le (minus (match_dup 1) (pc)) (const_int 128))
(eq (symbol_ref ("which_alternative")) (const_int 0)))
(const_int 2)
(const_int 8)))]
Thanks,
Revital
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