[Qemu-devel] [Bug 617528] Re: Incorrect translation of unary PPC/SPE instructions (efdneg etc.)

[Aurelien Jarno]

** Changed in: qemu
       Status: New => Fix Committed

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https://bugs.launchpad.net/bugs/617528

Title:
  Incorrect translation of unary PPC/SPE instructions (efdneg etc.)

Status in QEMU:
  Fix Committed

Bug description:
  The translation for the following PPC/SPE (e500) instructions is wrong in 
QEMU git 6cbf4c8c:

    evfsabs, evfsnabs, evfsneg
    efdabs, efdnabs, efdneg
    efsabs, efsnabs, efsneg

As you can see from the provided patch, these ought to write their result to 
the destination register (rD) and not modify the source register (rA) in-place.

It's rather hard to generate a test-case for this with GCC, since it likes to 
put the input and output of a unary operation into the same registers (that's 
probably also the reason why this went unnoticed). There is however a broken 
code path in the EGLIBC function for sin() when compiled for e500v2. It returns 
nonsense results for e.g. -1.0. Trivial test code follows:

    #include <stdio.h>
    #include <stdlib.h>
    #include <math.h>

    int main(int argc, char **argv)
    {
      double x = strtod(argv[1], NULL);
      printf("%.14g\n", sin(x));
      return 0;
    }

Result before the patch (WRONG):

$ qemu-ppc -cpu e500v2 sintest -1.0
-1

Result after the patch (OK):

$ qemu-ppc -cpu e500v2 sintest -1.0
-0.84147071838379

A self-contained test-case using inline assembler can be provided upon request.