Taking argument addresses.

[D. Towner]

Hi all,

I am currently porting gcc to a new target (16-bit DSP). Suppose I have
the following code:

  void fn(int a, int b) {
   int* ptr = &b;
  }

In order to move the address of `b' into a register, I can use the
following pattern:

  (define_insn "*movhi_address"
    [(set (match_operand:HI 0 "register_operand" "=r")
   	  (match_operand:HI 1 "address_operand" "r"))]
    ""
    "/* something. */")

However, if I try to take the address of `a' instead, the above pattern
fails (I get an error in instantiate_virtual_regs). I don't understand the
difference between taking the two addresses. Through trial and error, I
have discovered that if I can get around the problem by defining another
pattern:

  (define_insn "*movhi_address2"
    [(set (match_operand:HI 0 "memory_operand" "=m")
          (match_operand:HI 1 "address_operand" "r"))]
    ""
    "/* something. */")

...and everything then works as expected. Whilst I am happy that defining
the pair of patterns (actually combined into a single pattern) does the
job, I would like to understand why there needs to be two?

Thanks,

 Dan.

===============================================================================
Dan Towner
dant@picochip.com
picoChip Designs Ltd.
+44 (0) 7786 702589