Re: Compiler optimization of division and remainder

hbaker@netcom.com (Henry G. Baker) writes:
      On another newsgroup, a poster claimed that some compilers could
      optimize a program which used both a/b and a%b (both quotient and
      remainder with the _same_ arguments) in such a way that the compiler
      would perform only a single hardware division operation and arrange to
      use both the quotient and remainder from this single operation.


We do this on the x86 where the idiv instruction plonks the quotient
in AX and the remainder in DX. Most other processors either don't have
hardware support or only give you one of the values out of the divide
unit depending on source instruction.


One can play a similar game with other multiple-output instructions:
for example, one gets given both the sin and the cos out of a chordic
algorithm and in many circumstances sin(x) and cos(x) get used
together.


----------------------------------------------------------------------
Dave Lloyd Email: Dave@occl-cam.demon.co.uk
Oxford and Cambridge Compilers Ltd Phone: (44) 1223 572074
55 Brampton Rd, Cambridge CB1 3HJ, UK
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