[PATCH GCC]Fix PR57540, try to choose scaled_offset address mode when expanding array reference

[Eric Botcazou]

> As reported in pr57540, gcc chooses bad address mode, resulting in A)
> invariant part of address expression is not kept or hoisted; b) additional
> computation which should be encoded in address expression.  The reason is
> when gcc runs into "addr+offset" (which is invalid) during expanding, it
> pre-computes the entire address and accesses memory unit using "MEM[reg]".
> Yet we can force addr into register and try to generate "reg+offset" which
> is valid for targets like ARM.  By doing this, we can:
> 1) keep addr in loop invariant form and hoist it later;
> 2) saving additional computation by taking advantage of scaled addressing
> mode;

Does the invalid address not go through arm_legitimize_address from here?

      /* Perform machine-dependent transformations on X
	 in certain cases.  This is not necessary since the code
	 below can handle all possible cases, but machine-dependent
	 transformations can make better code.  */
      {
	rtx orig_x = x;
	x = targetm.addr_space.legitimize_address (x, oldx, mode, as);
	if (orig_x != x && memory_address_addr_space_p (mode, x, as))
	  goto done;
      }

-- 
Eric Botcazou