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.