[Qemu-devel] [Bug 1843205] [NEW] Inaccurate Fmod on i386

[Qemu-devel] [Bug 1843205] [NEW] Inaccurate Fmod on i386

[Alexander Huszagh]

Public bug reported:

# Qemu Version

```bash
$ qemu-i386 --version
qemu-i386 version 3.0.1 (qemu-3.0.1-4.fc29)
Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
```

# Failing Source Code (C)

```c
#include <math.h>
#include <stdio.h>

int main()
{
    double x = 29860476080414620.0;
    double y = 17.0;
    double z = fmod(x, y);
    printf("%f\n", z);
    return 0;
}
```

The code was compiled with GCC (8.3.1) on x86-64 with the flags `-O3
-m32 -lm -static`.

# Emitted (Annotated) Assembly

In order to facilitate debugging the issue, the following assembly was
generated to show nothing unusual occurred during compilation. The
assembly was generated with flags `-S -O3 -m32 -lm`, and then annotated
to show the operands to fmod.

```asm
	.file	"a.c"
	.text
	.section	.rodata.str1.1,"aMS",@progbits,1
.LC2:
	.string	"%f\n"
	.section	.text.startup,"ax",@progbits
	.p2align 4,,15
	.globl	main
	.type	main, @function
main:
.LFB16:
	.cfi_startproc
	leal	4(%esp), %ecx
	.cfi_def_cfa 1, 0
	andl	$-16, %esp
	pushl	-4(%ecx)
	pushl	%ebp
	.cfi_escape 0x10,0x5,0x2,0x75,0
	movl	%esp, %ebp
	pushl	%ecx
	.cfi_escape 0xf,0x3,0x75,0x7c,0x6
	subl	$4, %esp
	pushl	$1076953088				; high 32-bits of double for y
	pushl	$0 						; low 32-bits of double for y
	pushl	$1130005884				; high 32-bits of double for x
	pushl	$2003187687				; low 32-bits of double for x
	call	fmod
	movl	$.LC2, (%esp)
	fstpl	4(%esp)
	call	printf
	movl	-4(%ebp), %ecx
	.cfi_def_cfa 1, 0
	addl	$16, %esp
	xorl	%eax, %eax
	leave
	.cfi_restore 5
	leal	-4(%ecx), %esp
	.cfi_def_cfa 4, 4
	ret
	.cfi_endproc
.LFE16:
	.size	main, .-main
	.ident	"GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
	.section	.note.GNU-stack,"",@progbits
```

# Result

Running the compiled binary on x86_64 produces the expected value of
`15.000000`, while using `qemu-i386 <binary>` produces the unexpected
result of `-4.000000`.

This was tested against:

1. Qemu 3.0.1 for Fedora 29.
2. Qemu 4.1.0 built from source, downloaded from https://download.qemu.org/qemu-4.1.0.tar.xz
3. Qemu built-from-source against commit 90b1e3afd33226b6078fec6d77a18373712a975c.

# Building Qemu

Qemu built-from-source was compiled as follows:

```bash
mkdir build && cd build
../configure --disable-kvm --target-list="i386-linux-user"
make -j 5
```

# Results

All built versions of Qemu running the 32-bit failed to produce the
accurate result. Using qemu-x86_64 against an x86_64 binary built from
the same C source code produces correct results. Running the 32-bit
binary natively produces the correct result.

** Affects: qemu
     Importance: Undecided
         Status: New


** Tags: fmod

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Title:
  Inaccurate Fmod on i386

Status in QEMU:
  New

Bug description:
  # Qemu Version

  ```bash
  $ qemu-i386 --version
  qemu-i386 version 3.0.1 (qemu-3.0.1-4.fc29)
  Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
  ```

  # Failing Source Code (C)

  ```c
  #include <math.h>
  #include <stdio.h>

  int main()
  {
      double x = 29860476080414620.0;
      double y = 17.0;
      double z = fmod(x, y);
      printf("%f\n", z);
      return 0;
  }
  ```

  The code was compiled with GCC (8.3.1) on x86-64 with the flags `-O3
  -m32 -lm -static`.

  # Emitted (Annotated) Assembly

  In order to facilitate debugging the issue, the following assembly was
  generated to show nothing unusual occurred during compilation. The
  assembly was generated with flags `-S -O3 -m32 -lm`, and then
  annotated to show the operands to fmod.

  ```asm
  	.file	"a.c"
  	.text
  	.section	.rodata.str1.1,"aMS",@progbits,1
  .LC2:
  	.string	"%f\n"
  	.section	.text.startup,"ax",@progbits
  	.p2align 4,,15
  	.globl	main
  	.type	main, @function
  main:
  .LFB16:
  	.cfi_startproc
  	leal	4(%esp), %ecx
  	.cfi_def_cfa 1, 0
  	andl	$-16, %esp
  	pushl	-4(%ecx)
  	pushl	%ebp
  	.cfi_escape 0x10,0x5,0x2,0x75,0
  	movl	%esp, %ebp
  	pushl	%ecx
  	.cfi_escape 0xf,0x3,0x75,0x7c,0x6
  	subl	$4, %esp
  	pushl	$1076953088				; high 32-bits of double for y
  	pushl	$0 						; low 32-bits of double for y
  	pushl	$1130005884				; high 32-bits of double for x
  	pushl	$2003187687				; low 32-bits of double for x
  	call	fmod
  	movl	$.LC2, (%esp)
  	fstpl	4(%esp)
  	call	printf
  	movl	-4(%ebp), %ecx
  	.cfi_def_cfa 1, 0
  	addl	$16, %esp
  	xorl	%eax, %eax
  	leave
  	.cfi_restore 5
  	leal	-4(%ecx), %esp
  	.cfi_def_cfa 4, 4
  	ret
  	.cfi_endproc
  .LFE16:
  	.size	main, .-main
  	.ident	"GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
  	.section	.note.GNU-stack,"",@progbits
  ```

  # Result

  Running the compiled binary on x86_64 produces the expected value of
  `15.000000`, while using `qemu-i386 <binary>` produces the unexpected
  result of `-4.000000`.

  This was tested against:

  1. Qemu 3.0.1 for Fedora 29.
  2. Qemu 4.1.0 built from source, downloaded from https://download.qemu.org/qemu-4.1.0.tar.xz
  3. Qemu built-from-source against commit 90b1e3afd33226b6078fec6d77a18373712a975c.

  # Building Qemu

  Qemu built-from-source was compiled as follows:

  ```bash
  mkdir build && cd build
  ../configure --disable-kvm --target-list="i386-linux-user"
  make -j 5
  ```

  # Results

  All built versions of Qemu running the 32-bit failed to produce the
  accurate result. Using qemu-x86_64 against an x86_64 binary built from
  the same C source code produces correct results. Running the 32-bit
  binary natively produces the correct result.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1843205/+subscriptions

[Qemu-devel] [Bug 1843205] Re: Inaccurate Fmod on i386

[Alexander Huszagh]

** Tags removed: fmod
** Tags added: float libm

-- 
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https://bugs.launchpad.net/bugs/1843205

Title:
  Inaccurate Fmod on i386

Status in QEMU:
  New

Bug description:
  # Qemu Version

  ```bash
  $ qemu-i386 --version
  qemu-i386 version 3.0.1 (qemu-3.0.1-4.fc29)
  Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
  ```

  # Failing Source Code (C)

  ```c
  #include <math.h>
  #include <stdio.h>

  int main()
  {
      double x = 29860476080414620.0;
      double y = 17.0;
      double z = fmod(x, y);
      printf("%f\n", z);
      return 0;
  }
  ```

  The code was compiled with GCC (8.3.1) on x86-64 with the flags `-O3
  -m32 -lm -static`.

  # Emitted (Annotated) Assembly

  In order to facilitate debugging the issue, the following assembly was
  generated to show nothing unusual occurred during compilation. The
  assembly was generated with flags `-S -O3 -m32 -lm`, and then
  annotated to show the operands to fmod.

  ```asm
  	.file	"a.c"
  	.text
  	.section	.rodata.str1.1,"aMS",@progbits,1
  .LC2:
  	.string	"%f\n"
  	.section	.text.startup,"ax",@progbits
  	.p2align 4,,15
  	.globl	main
  	.type	main, @function
  main:
  .LFB16:
  	.cfi_startproc
  	leal	4(%esp), %ecx
  	.cfi_def_cfa 1, 0
  	andl	$-16, %esp
  	pushl	-4(%ecx)
  	pushl	%ebp
  	.cfi_escape 0x10,0x5,0x2,0x75,0
  	movl	%esp, %ebp
  	pushl	%ecx
  	.cfi_escape 0xf,0x3,0x75,0x7c,0x6
  	subl	$4, %esp
  	pushl	$1076953088				; high 32-bits of double for y
  	pushl	$0 						; low 32-bits of double for y
  	pushl	$1130005884				; high 32-bits of double for x
  	pushl	$2003187687				; low 32-bits of double for x
  	call	fmod
  	movl	$.LC2, (%esp)
  	fstpl	4(%esp)
  	call	printf
  	movl	-4(%ebp), %ecx
  	.cfi_def_cfa 1, 0
  	addl	$16, %esp
  	xorl	%eax, %eax
  	leave
  	.cfi_restore 5
  	leal	-4(%ecx), %esp
  	.cfi_def_cfa 4, 4
  	ret
  	.cfi_endproc
  .LFE16:
  	.size	main, .-main
  	.ident	"GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
  	.section	.note.GNU-stack,"",@progbits
  ```

  # Result

  Running the compiled binary on x86_64 produces the expected value of
  `15.000000`, while using `qemu-i386 <binary>` produces the unexpected
  result of `-4.000000`.

  This was tested against:

  1. Qemu 3.0.1 for Fedora 29.
  2. Qemu 4.1.0 built from source, downloaded from https://download.qemu.org/qemu-4.1.0.tar.xz
  3. Qemu built-from-source against commit 90b1e3afd33226b6078fec6d77a18373712a975c.

  # Building Qemu

  Qemu built-from-source was compiled as follows:

  ```bash
  mkdir build && cd build
  ../configure --disable-kvm --target-list="i386-linux-user"
  make -j 5
  ```

  # Results

  All built versions of Qemu running the 32-bit failed to produce the
  accurate result. Using qemu-x86_64 against an x86_64 binary built from
  the same C source code produces correct results. Running the 32-bit
  binary natively produces the correct result.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1843205/+subscriptions

[Bug 1843205] Re: Inaccurate Fmod on i386

[Peter Maydell]

On current head-of-git QEMU I get the correct answer. I think this was
probably fixed by the reimplementation of the 'fprem' emulation in
commit 5ef396e2ba86, which was in the 5.1.0 release.


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

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1843205

Title:
  Inaccurate Fmod on i386

Status in QEMU:
  Fix Released

Bug description:
  # Qemu Version

  ```bash
  $ qemu-i386 --version
  qemu-i386 version 3.0.1 (qemu-3.0.1-4.fc29)
  Copyright (c) 2003-2017 Fabrice Bellard and the QEMU Project developers
  ```

  # Failing Source Code (C)

  ```c
  #include <math.h>
  #include <stdio.h>

  int main()
  {
      double x = 29860476080414620.0;
      double y = 17.0;
      double z = fmod(x, y);
      printf("%f\n", z);
      return 0;
  }
  ```

  The code was compiled with GCC (8.3.1) on x86-64 with the flags `-O3
  -m32 -lm -static`.

  # Emitted (Annotated) Assembly

  In order to facilitate debugging the issue, the following assembly was
  generated to show nothing unusual occurred during compilation. The
  assembly was generated with flags `-S -O3 -m32 -lm`, and then
  annotated to show the operands to fmod.

  ```asm
  	.file	"a.c"
  	.text
  	.section	.rodata.str1.1,"aMS",@progbits,1
  .LC2:
  	.string	"%f\n"
  	.section	.text.startup,"ax",@progbits
  	.p2align 4,,15
  	.globl	main
  	.type	main, @function
  main:
  .LFB16:
  	.cfi_startproc
  	leal	4(%esp), %ecx
  	.cfi_def_cfa 1, 0
  	andl	$-16, %esp
  	pushl	-4(%ecx)
  	pushl	%ebp
  	.cfi_escape 0x10,0x5,0x2,0x75,0
  	movl	%esp, %ebp
  	pushl	%ecx
  	.cfi_escape 0xf,0x3,0x75,0x7c,0x6
  	subl	$4, %esp
  	pushl	$1076953088				; high 32-bits of double for y
  	pushl	$0 						; low 32-bits of double for y
  	pushl	$1130005884				; high 32-bits of double for x
  	pushl	$2003187687				; low 32-bits of double for x
  	call	fmod
  	movl	$.LC2, (%esp)
  	fstpl	4(%esp)
  	call	printf
  	movl	-4(%ebp), %ecx
  	.cfi_def_cfa 1, 0
  	addl	$16, %esp
  	xorl	%eax, %eax
  	leave
  	.cfi_restore 5
  	leal	-4(%ecx), %esp
  	.cfi_def_cfa 4, 4
  	ret
  	.cfi_endproc
  .LFE16:
  	.size	main, .-main
  	.ident	"GCC: (GNU) 8.3.1 20190223 (Red Hat 8.3.1-2)"
  	.section	.note.GNU-stack,"",@progbits
  ```

  # Result

  Running the compiled binary on x86_64 produces the expected value of
  `15.000000`, while using `qemu-i386 <binary>` produces the unexpected
  result of `-4.000000`.

  This was tested against:

  1. Qemu 3.0.1 for Fedora 29.
  2. Qemu 4.1.0 built from source, downloaded from https://download.qemu.org/qemu-4.1.0.tar.xz
  3. Qemu built-from-source against commit 90b1e3afd33226b6078fec6d77a18373712a975c.

  # Building Qemu

  Qemu built-from-source was compiled as follows:

  ```bash
  mkdir build && cd build
  ../configure --disable-kvm --target-list="i386-linux-user"
  make -j 5
  ```

  # Results

  All built versions of Qemu running the 32-bit failed to produce the
  accurate result. Using qemu-x86_64 against an x86_64 binary built from
  the same C source code produces correct results. Running the 32-bit
  binary natively produces the correct result.

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1843205/+subscriptions