[Bug 1728325] Re: POWER8: Wrong behaviour with float-to-int punning

[Thomas Huth]

The QEMU project is currently considering to move its bug tracking to another 
system. For this we need to know which bugs are still valid and which could be 
closed already. Thus we are setting older bugs to "Incomplete" now.
If you still think this bug report here is valid, then please switch the state 
back to "New" within the next 60 days, otherwise this report will be marked as 
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** Changed in: qemu
       Status: New => Incomplete

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https://bugs.launchpad.net/bugs/1728325

Title:
  POWER8: Wrong behaviour with float-to-int punning

Status in QEMU:
  Incomplete

Bug description:
  Building a reduced test program with 'gcc -O2 -fno-inline
  -mcpu=power8' produces wrong results at runtime. I don't think gcc is
  at fault here.

  ---
  #include <stdio.h>

  int getWord(const float x)
  {
    return *(int*)&x;
  }

  void main()
  {
      int foo = getWord(+123.456f);
      int bar = getWord(-123.456f);

      printf("%d\n", foo);
      printf("%d\n", bar);
      return;
  }
  ---

  This prints:
  ---
  0
  0
  ---

  Compiling with 'gcc -O2 -fno-inline -mcpu=power7' and you instead get the 
expected result:
  ---
  1123477881
  -1024005767
  ---

  
  The different between the two programs is:

  --- power7.s
  +++ power8.s
  @@ -6,9 +6,9 @@
        .globl getWord
        .type   getWord, @function
   getWord:
  -     stfs 1,-16(1)
  -     ori 2,2,0
  -     lwa 3,-16(1)
  +     xscvdpspn 0,1
  +     mfvsrwz 3,0
  +     extsw 3,3
        blr
        .long 0
        .byte 0,0,0,0,0,0,0,0
          .size   getWord,.-getWord

  
  Seems like qemu doesn't handle xscvdpspn/mfvsrwz correctly.

  https://github.com/qemu/qemu/commit/7ee19fb9d682689d36c849576c808cf92e3bae40
  https://github.com/qemu/qemu/commit/f5c0f7f981333da59cc35c3210d05ec1775c97c1

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