ppc instructions in gas

Ian Lance Taylor ian@airs.com
Thu May 6 20:06:00 GMT 1999


   Date: Thu, 6 May 1999 15:07:24 -0700
   From: Richard Henderson <rth@cygnus.com>

   On Wed, May 05, 1999 at 04:52:32PM -0600, Cort Dougan wrote:
   > The signed change at the top makes sense with what the manual says.  The
   > operand in questions should be a 16-bit unsigned immediate value not signed
   > (as it was before).
   [...]
   >    /* The SI field in a D form instruction when we accept a wide range
   >       of positive values.  */
   >  #define SISIGNOPT SI + 1
   > -  { 16, 0, 0, 0, PPC_OPERAND_SIGNED | PPC_OPERAND_SIGNOPT },
   > +  { 16, 0, 0, 0, /*PPC_OPERAND_SIGNED |*/ PPC_OPERAND_SIGNOPT },

   Looking at this again, I wonder if this change is correct.

   Which instruction were you trying to fix?  "liu" perhaps? 
   As-is, it affects instructions like "addis", and you can't
   really add +50000, can you?

We specifically want to accept a wide range of values, because users
in practice use a wide range of values.  That is, users in practice
use
    lis  4,-0x8000
    lis  4,0x7fff
    lis  4,0xffff
    lis  4,-1
and expect them all to work.  It's true that the PowerPC docs prohibit
the last one.  However, people use it in real code, so gas accepts
it.  The last two instructions above generate the same bytes.

I don't have the context of the original message.  The addis/lis
instruction is documented to take a signed operand.  I don't know why
it makes sense to remove PPC_OPERAND_SIGNED.

Ian


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