Re: [PATCH] [MSP430] [PR78554] Prevent SUBREG from referencing a SYMBOL_REF

[Jeff Law <law at redhat dot com>]

On 08/24/2017 07:18 AM, Jozef Lawrynowicz wrote:
> As reported in PR78554, attempting to store an __int20 address in memory
> causes an ICE due to an invalid insn. This only occurs at optimisation
> levels higher than -O0 because these optimisation levels pass
> -ftree-ter, which causes the compiler to try and do the store in
> one instruction.
> The issue in the insn is that a SUBREG references a SYMBOL_REF.
> 
> I guess the compiler gets into this situation because it assumes that
> it can execute a move instruction where both src and dst are in memory,
> but this isn't possible with __int20.
GCC doesn't really make this assumption, but it does rely on the
target's movXX expanders to handle generating correct code for a
mem->mem move.

Commonly on risc targets what you'll see is a movXX expanders like this:

(define_expand "movsi"
  [(set (match_operand:SI 0 "nonimmediate_operand")
        (match_operand:SI 1 "general_operand"))]
  ""
{
  /* One of the ops has to be in a register.  */
  if (!register_operand (operand1, SImode)
      && !register_operand (operand0, SImode))
    operands[1] = force_reg (SImode, operand1);



> 
> The attached patch prevents a instance of SUBREG being created where the
> subword is a SYMBOL_REF.
> 
> If the patch is acceptable, I would appreciate if someone could commit
> it for me, as I do not have write access.
I was going to look deeper at this, but can't as the trunk currently
aborts in init_derived_machine_modes when compiling the testcase.
Presumably something about Richard S's work is busted when it comes to
dealing with partial-word stuff.

I'll ping Richard S. to take a looksie.

Jeff