ldr instruction selection in the aarch64 backend

[Virendra Kumar Pathak]

Hi Linaro Toolchain Group,

I have a question on the ldr instruction selection in the aarch64 backend.
Could someone help me in this regards, please?

I am trying to allow only type A instructions while disabling the type B.
Type A example: ldr x4, [x20,x1] ---> allow
Type B example: ldr x1, [x9,x3,lsl #3] ---> disable


Experiment/My Understanding -
aarch64_classify_address() returns true if rtx X is a valid address. If
allow_reg_index_p=true then it calls aarch64_classify_index().
aarch64_classify_index() identify the address mode of second operand (op1)
and accordingly calculate the shift.
If shift=0 then type A is generated otherwise Type B will be generated.

Thus if (shift != 0) then I am returning 'false' from
aarch64_classify_index().
-------------------------patch---------
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -3586,6 +3586,9 @@ aarch64_classify_index (struct aarch64_address_info
*info, rtx x,
if (GET_CODE (index) == SUBREG)
index = SUBREG_REG (index);
+ if (shift != 0)
+ return false;
if ((shift == 0 ||
(shift > 0 && shift <= 3
&& (1 << shift) == GET_MODE_SIZE (mode)))
---------------------------------------

Result -
Before change
ldr x0, [x13,x0,lsl #3]
After Change
lsl x1, x1, #3
ldr x0, [x15,x1]

Question -
How the returning 'false' from aarch64_classify_index() is resulting in the
selection of type A versus type B?
I could not find the function which is taking the decision based on return
from aarch64_classify_address().
Could someone please explain this process or point me to the relevant files
or code?
Please correct me if my understanding is wrong.

Thanks in advance for your time and patience.

[Pinski, Andrew]

aarch64_legitimate_address_hook_p is the place where the result of aarch64_classify_address is returned to the middle-end. The middle-end then knows that possibility for a+b is a legitimate address so it forces x3 << 3 into a register and tries aarch64_legitimate_address_hook_p again.

Thanks,
Andrew Pinski

From: linaro-toolchain [mailto:linaro-toolchain-***@lists.linaro.org] On Behalf Of Virendra Kumar Pathak
Sent: Friday, February 5, 2016 6:58 AM
To: Linaro Toolchain Mailman List <linaro-***@lists.linaro.org>
Subject: ldr instruction selection in the aarch64 backend

Hi Linaro Toolchain Group,

I have a question on the ldr instruction selection in the aarch64 backend.
Could someone help me in this regards, please?

I am trying to allow only type A instructions while disabling the type B.
Type A example: ldr x4, [x20,x1] ---> allow
Type B example: ldr x1, [x9,x3,lsl #3] ---> disable


Experiment/My Understanding -
aarch64_classify_address() returns true if rtx X is a valid address. If allow_reg_index_p=true then it calls aarch64_classify_index().
aarch64_classify_index() identify the address mode of second operand (op1) and accordingly calculate the shift.
If shift=0 then type A is generated otherwise Type B will be generated.

Thus if (shift != 0) then I am returning 'false' from aarch64_classify_index().
-------------------------patch---------
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -3586,6 +3586,9 @@ aarch64_classify_index (struct aarch64_address_info *info, rtx x,
if (GET_CODE (index) == SUBREG)
index = SUBREG_REG (index);
+ if (shift != 0)
+ return false;
if ((shift == 0 ||
(shift > 0 && shift <= 3
&& (1 << shift) == GET_MODE_SIZE (mode)))
---------------------------------------

Result -
Before change
ldr x0, [x13,x0,lsl #3]
After Change
lsl x1, x1, #3
ldr x0, [x15,x1]

Question -
How the returning 'false' from aarch64_classify_index() is resulting in the selection of type A versus type B?
I could not find the function which is taking the decision based on return from aarch64_classify_address().
Could someone please explain this process or point me to the relevant files or code?
Please correct me if my understanding is wrong.

Thanks in advance for your time and patience.

[Virendra Kumar Pathak]

Hi Andrew,

Thanks for explaining the process.

On 5 February 2016 at 21:49, Pinski, Andrew <

Post by Pinski, Andrew
aarch64_legitimate_address_hook_p is the place where the result of
aarch64_classify_address is returned to the middle-end. The middle-end
then knows that possibility for a+b is a legitimate address so it forces x3
<< 3 into a register and tries aarch64_legitimate_address_hook_p again.
Thanks,
Andrew Pinski
*On Behalf Of *Virendra Kumar Pathak
*Sent:* Friday, February 5, 2016 6:58 AM
*Subject:* ldr instruction selection in the aarch64 backend
Hi Linaro Toolchain Group,
I have a question on the ldr instruction selection in the aarch64 backend.
Could someone help me in this regards, please?
I am trying to allow only type A instructions while disabling the type B.
Type A example: ldr x4, [x20,x1] ---> allow
Type B example: ldr x1, [x9,x3,lsl #3] ---> disable
Experiment/My Understanding -
aarch64_classify_address() returns true if rtx X is a valid address. If
allow_reg_index_p=true then it calls aarch64_classify_index().
aarch64_classify_index() identify the address mode of second operand (op1)
and accordingly calculate the shift.
If shift=0 then type A is generated otherwise Type B will be generated.
Thus if (shift != 0) then I am returning 'false' from
aarch64_classify_index().
-------------------------patch---------
--- a/gcc/config/aarch64/aarch64.c
+++ b/gcc/config/aarch64/aarch64.c
@@ -3586,6 +3586,9 @@ aarch64_classify_index (struct aarch64_address_info *info, rtx x,
if (GET_CODE (index) == SUBREG)
index = SUBREG_REG (index);
+ if (shift != 0)
+ return false;
if ((shift == 0 ||
(shift > 0 && shift <= 3
&& (1 << shift) == GET_MODE_SIZE (mode)))
---------------------------------------
Result -
Before change
ldr x0, [x13,x0,lsl #3]
After Change
lsl x1, x1, #3
ldr x0, [x15,x1]
Question -
How the returning 'false' from aarch64_classify_index() is resulting in
the selection of type A versus type B?
I could not find the function which is taking the decision based on return
from aarch64_classify_address().
Could someone please explain this process or point me to the relevant files or code?
Please correct me if my understanding is wrong.
Thanks in advance for your time and patience.
--
with regards,
Virendra Kumar Pathak