[Bug rtl-optimization/17104] Non-optimal code generation for bitfield initialization

["mmitchel at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Additional Comments From mmitchel at gcc dot gnu dot org  2004-09-20 01:56 -------
In 32-bit mode the initial RTL looks like:

  (set:SI (reg:SI 124) (mem:SI (reg:SI 122)))
  ...
  (set:SI (mem:SI (reg:SI 122)) (reg:SI 125))

CSE recognizes that at this point the MEM and REG 125 contain the same value,
and therefore replaces the next read of the MEM with REG 125.

In 64-bit mode, the RTL looks like:

  (set:DI (reg:DI 125) (zero_extend:DI (mem:SI (reg:DI 123))))
  ...
  (set:SI (mem:SI (reg:SI 123)) (reg:SI 127))

The zero-extends applied to the load prevent CSE from recognizing that the MEM
written and the MEM read are the same.

There are several possible approaches to consider: 

  (a) Modify the PowerPC machine description to emit an SImode load,
      instead of the zero-extend sequence.  (The hardware instruction
      actually *is* zero-extending, which is presumably why the
      machine description generates the zero-extension.)

  (b) Modify the PowerPC machine description to emit the
      zero-extension as a separate instruction.  (And hope that the
      compiler will combine them into one instruction later if
      possible.)

  (c) Modify CSE to recognize the zero_extend idiom and replace the
      operand to the zero_extend.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17104