Re: mips-tdep.c: Sign-extend pointers for n32

["Maciej W. Rozycki" <macro@mips.com>]

On Wed, 19 Dec 2007, Daniel Jacobowitz wrote:

> What happens to an 8-bit unsigned char?  How about an 8-bit signed
> char?

 Hmm, you have made me wonder...  Obviously 32-bit integer quantities are 
special -- they are always sign-extended so that 32-bit ALU operations may 
be used on them.  There is this statement in the ABI document:

"All integer parameters are promoted (that is, sign- or zero-extended to 
64-bit integers and passed in a single register). Typically, no code is 
required for the promotion."

This note about no code requirement may be misleading -- this may be true 
for the run time, because the use of "lb/lbu/lh/lhu" as appropriate would 
have had the effect of doing the correct extension when the value was 
fetched into a register originally, but here we are in a different 
position as it is GDB that is the caller.  Let me dig through the 
document...  Nothing relevant apparently found.

 But in practice it should not matter -- however you represent 8-bit and 
16-bit quantities you cannot overflow into the 64-bit data space as with a 
flip of the bit #31 the upper 32 bits follow and when a result is written 
back to memory or is otherwise finally processed (like output in a textual 
form) it has to be masked to its data width again (obviously "sb/sh" do 
this implicitly).  So I believe the change is correct as it is and the 
question is academic.  Let me know if you think otherwise.

> What ABI is that?  I thought none of the ELF targets used n32 or n64,
> but maybe SDE is an exception.

 N32, as documented in "MIPSpro N32 ABI Handbook" from SGI.  I can chase a 
link if you cannot locate the PDF.  The SDE target uses it as the default 
64-bit ABI implied by the "-mips64" switch.

  Maciej