[Qemu-devel] [Bug 1821444] [NEW] qemu-ppc (user) incorrectly translates float32 arithmetics

[Sergei Trofimovich]

Public bug reported:

I'm using qemu-3.1.0 (Gentoo).

When I was running regression test suite via qemu-ppc for GHC I noticed
a few uint32_t<->float32 failures I did not expect to encounter.

Here is an example

$ cat a.c
#include <stdio.h>
#include <stdint.h>

int main() {
    volatile uint32_t i = 1;
    printf("0x1 = %e\n", *(volatile float*)&i);
}

$ powerpc-unknown-linux-gnu-gcc -O2 a.c -Wall -o a -fno-strict-aliasing 
-fno-stack-protector -static && ./a
0x1 = 2.802597e-45

$ scp a timberdoodle.ppc64.dev.gentoo.org:~/
a                                                                               
                        100%  826KB 102.0KB/s   00:08    

$ ssh timberdoodle.ppc64.dev.gentoo.org ./a
0x1 = 1.401298e-45
$ qemu-ppc ./a
0x1 = 2.802597e-45

Looks like off-by-one bit somewhere. I'm not sure if it's FPU
instruction or some internals of printf() that are emulated incorrectly.

** Affects: qemu
     Importance: Undecided
         Status: New

** Attachment added: "statically linked example"
   https://bugs.launchpad.net/bugs/1821444/+attachment/5248800/+files/a-ppc

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https://bugs.launchpad.net/bugs/1821444

Title:
  qemu-ppc (user) incorrectly translates float32 arithmetics

Status in QEMU:
  New

Bug description:
  I'm using qemu-3.1.0 (Gentoo).

  When I was running regression test suite via qemu-ppc for GHC I
  noticed a few uint32_t<->float32 failures I did not expect to
  encounter.

  Here is an example

  $ cat a.c
  #include <stdio.h>
  #include <stdint.h>

  int main() {
      volatile uint32_t i = 1;
      printf("0x1 = %e\n", *(volatile float*)&i);
  }

  $ powerpc-unknown-linux-gnu-gcc -O2 a.c -Wall -o a -fno-strict-aliasing 
-fno-stack-protector -static && ./a
  0x1 = 2.802597e-45

  $ scp a timberdoodle.ppc64.dev.gentoo.org:~/
  a                                                                             
                          100%  826KB 102.0KB/s   00:08    

  $ ssh timberdoodle.ppc64.dev.gentoo.org ./a
  0x1 = 1.401298e-45
  $ qemu-ppc ./a
  0x1 = 2.802597e-45

  Looks like off-by-one bit somewhere. I'm not sure if it's FPU
  instruction or some internals of printf() that are emulated
  incorrectly.

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