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Date: Tue, 31 Oct 2017 09:35:55 -0000
From: Richard Henderson <rth@twiddle.net>
Reply-To: Bug 1728325 <1728325@bugs.launchpad.net>
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Subject: [Qemu-devel] [Bug 1728325] Re: POWER8: Wrong behaviour with
 float-to-int punning
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To: qemu-devel@nongnu.org

This is buggy C.

https://gcc.gnu.org/bugs/#nonbugs_c

** Changed in: qemu
       Status: New =3D> Invalid

-- =

You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1728325

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

Status in QEMU:
  Invalid

Bug description:
  Building a reduced test program with 'gcc -O2 -fno-inline
  -mcpu=3Dpower8' 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 =3D getWord(+123.456f);
      int bar =3D getWord(-123.456f);

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

  This prints:
  ---
  0
  0
  ---

  Compiling with 'gcc -O2 -fno-inline -mcpu=3Dpower7' and you instead get t=
he 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/7ee19fb9d682689d36c849576c808cf92e3ba=
e40
  https://github.com/qemu/qemu/commit/f5c0f7f981333da59cc35c3210d05ec1775c9=
7c1

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