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Re: df.c and partial writes/REG_EQUAL notes


On Wed, 24 Oct 2001 wrote:

> Right.  Review the movxx section in the Machine Descriptions part of the
> manual.  Quoting:
> @cindex @code{mov@var{m}} instruction pattern
> @item @samp{mov@var{m}}
> Here @var{m} stands for a two-letter machine mode name, in lower case.
> This instruction pattern moves data with that machine mode from operand
> 1 to operand 0.  For example, @samp{movsi} moves full-word data.
> If operand 0 is a @code{subreg} with mode @var{m} of a register whose
> own mode is wider than @var{m}, the effect of this instruction is
> to store the specified value in the part of the register that corresponds
> to mode @var{m}.  The effect on the rest of the register is undefined.
> Based on that I believe there should be no dependency since insn #2
> leaves the rest of the DI register undefined.

No, that's exactly the problem I was asking some months before about this
documentation.  Only the _word_ containing the subreg is touched by such a
move, i.e. if the subreg describes a complete word, it leaves no bits
undefined.  Depending on the machine a (set (subreg:SI (reg:DI x) 0))
clobbers the complete (reg:DI x) (if it only takes one hardreg), or only
the lower half.


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