Aligning stack offsets for spills

Peter Bergner
Thu Jun 10 19:28:34 GMT 2021

On 6/7/21 2:00 PM, Jeff Law wrote:
> I can't divulge many of the details right now, but one of the quirks of our
> architecture is that reg+d addressing modes for our vector loads/stores require
> the displacement to be aligned.  This is an artifact of how these instructions
> are encoded.

Given what you're describing, it sounds like POWER has something similar.
Our reg+displacement addressing uses 16-bit displacements using D, DS or DQ
operand fields.  The D field encodes the entire 16-bits, but the DS and DQ
fields only encode 14-bits and 12-bits respectively.  The DS and DQ operands
have the same maximum displacement as D operands, we just force that their
bottom 2-bits/4-bits must be zero, so we don't need to include them in the
insn encoding.  I believe this is all just handled in our legitimate address
routines, but maybe Segher and/or Mike can correct me if I'm wrong?


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