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ryan baird

2012-Aug-06 19:43 UTC

[LLVMdev] Casting from float to unsigned char - incorrect output?

I am compiling the following code for the MIPS architecture:

unsigned char trunc(float f) {
        return (unsigned char) f;
}

and it produces the following assembly (directives removed for convenience:
trunc:
    trunc.w.s    $f0, $f12
    mfc1    $2, $f0
    jr    $ra
    nop

However, this does not seem to produce the correct output for negative
numbers. When I run the following code, I get -1 instead of 255 (which is
produced by compiling natively with gcc).
int trunc(float c);
int main() {
  printf("%d\n", trunc(-1.0));
}

I am running the mips code on a PISA simulator (SimpleScalar's  Simple-Sim
3.0) instead of a MIPS IV simulator, so there is a little bit of
translation occurring before I can simulate it; here is the revised code:
trunc:
    cvt.w.s    $f0, $f12
    mfc1    $2, $f0
    jr    $ra
    nop
the cvt.w.s function in PISA MIPS is rounding towards zero, so it should
meet the specification for trunc.w.s in MIPS IV.

Here are the commands I used to compile test.c (with the trunc function in
it):
clang -emit-llvm -mfloat-abi=hard -ccc-host-triple mipsel-unknown-linux
-ccc-clang-archs mipsel -O3 -S -o test_unopt.ll test.c
opt -std-compile-opts test_unopt.ll -o test.ll
llc -march=mipsel -mcpu=mips32 -float-abi=hard -relocation-model=static
test.ll -o test.s

Here is the llvm intermediate representation:
; ModuleID = 'test_unopt.ll'
target datalayout
"e-p:32:32:32-i1:8:8-i8:8:32-i16:16:32-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-n32"
target triple = "mipsel-unknown-linux"

define zeroext i8 @trunc(float %f) nounwind readnone {
entry:
  %conv = fptoui float %f to i8
  ret i8 %conv
}

This is the assembly produced by another compiler; it is more complex, but
it produces the expected output, matching with GCC: (directives removed for
convenience)
trunc:
        mov.s   $f4,$f12
        la      $2,L6
        l.s     $f6,($2)
        c.lt.s  $f12,$f6
        bc1t    .L3
        sub.s   $f0,$f12,$f6
        cvt.w.s $f0,$f0
        mfc1    $2,$f0
        lui     $3,32768
        addu    $3,$2,$3
        j       .L4
.L3:
         cvt.w.s $f4,$f4
         mfc1    $3,$f4
.L4:
        sll     $2,$3,24
        srl     $2,$2,24
        sll     $2,$2,24
        srl     $2,$2,24
        j       $31
.data
.align 4
L6:
.word 1325400064

Am I correct in my analysis that LLVM's assembly output is wrong?  Is there
a way for me to get the correct output?
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Eli Friedman

2012-Aug-06 20:03 UTC

head link

[LLVMdev] Casting from float to unsigned char - incorrect output?

On Mon, Aug 6, 2012 at 12:43 PM, ryan baird <ryanrbaird at gmail.com>
wrote:> I am compiling the following code for the MIPS architecture:
>
> unsigned char trunc(float f) {
>         return (unsigned char) f;
> }
>
> and it produces the following assembly (directives removed for convenience:
> trunc:
>     trunc.w.s    $f0, $f12
>     mfc1    $2, $f0
>     jr    $ra
>     nop
>
> However, this does not seem to produce the correct output for negative
> numbers. When I run the following code, I get -1 instead of 255 (which is
> produced by compiling natively with gcc).
> int trunc(float c);
> int main() {
>   printf("%d\n", trunc(-1.0));
> }
That code has undefined behavior; see 6.3.1.4p1 in C99

-Eli

Jim Grosbach

2012-Aug-06 20:04 UTC

head link

[LLVMdev] Casting from float to unsigned char - incorrect output?

On Aug 6, 2012, at 12:43 PM, ryan baird <ryanrbaird at gmail.com> wrote:
> I am compiling the following code for the MIPS architecture: 
> 
> unsigned char trunc(float f) {
>         return (unsigned char) f;
> }
> 
> and it produces the following assembly (directives removed for convenience:
> trunc:
>     trunc.w.s    $f0, $f12
>     mfc1    $2, $f0
>     jr    $ra
>     nop
> 
> However, this does not seem to produce the correct output for negative
numbers. When I run the following code, I get -1 instead of 255 (which is
produced by compiling natively with gcc).
> int trunc(float c);
> int main() {
>   printf("%d\n", trunc(-1.0));
> }
The prototype for trunc() here and it's definition above don't match.
Dunno if that's related to the behavior you're seeing, but it's
definitely a bug in the source code.
> 
> I am running the mips code on a PISA simulator (SimpleScalar's 
Simple-Sim 3.0) instead of a MIPS IV simulator, so there is a little bit of
translation occurring before I can simulate it; here is the revised code:
> trunc:
>     cvt.w.s    $f0, $f12
>     mfc1    $2, $f0
>     jr    $ra
>     nop
> the cvt.w.s function in PISA MIPS is rounding towards zero, so it should
meet the specification for trunc.w.s in MIPS IV.
> 
> Here are the commands I used to compile test.c (with the trunc function in
it):
> clang -emit-llvm -mfloat-abi=hard -ccc-host-triple mipsel-unknown-linux
-ccc-clang-archs mipsel -O3 -S -o test_unopt.ll test.c
> opt -std-compile-opts test_unopt.ll -o test.ll
> llc -march=mipsel -mcpu=mips32 -float-abi=hard -relocation-model=static
test.ll -o test.s
> 
> Here is the llvm intermediate representation:
> ; ModuleID = 'test_unopt.ll'
> target datalayout =
"e-p:32:32:32-i1:8:8-i8:8:32-i16:16:32-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-n32"
> target triple = "mipsel-unknown-linux"
> 
> define zeroext i8 @trunc(float %f) nounwind readnone {
> entry:
>   %conv = fptoui float %f to i8
>   ret i8 %conv
> }
> 
> This is the assembly produced by another compiler; it is more complex, but
it produces the expected output, matching with GCC: (directives removed for
convenience)
> trunc:
>         mov.s   $f4,$f12
>         la      $2,L6
>         l.s     $f6,($2)
>         c.lt.s  $f12,$f6
>         bc1t    .L3
>         sub.s   $f0,$f12,$f6
>         cvt.w.s $f0,$f0
>         mfc1    $2,$f0
>         lui     $3,32768
>         addu    $3,$2,$3
>         j       .L4
> .L3:
>          cvt.w.s $f4,$f4
>          mfc1    $3,$f4
> .L4:
>         sll     $2,$3,24
>         srl     $2,$2,24
>         sll     $2,$2,24 
>         srl     $2,$2,24  
>         j       $31
> .data
> .align 4
> L6:
> .word 1325400064
> 
> Am I correct in my analysis that LLVM's assembly output is wrong?  Is
there a way for me to get the correct output?
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

ryan baird

2012-Aug-06 20:28 UTC

head link

[LLVMdev] Casting from float to unsigned char - incorrect output?

>
> I didn't realize the code was undefined, I'll let my professor
know; but
> for code comparison purposes we're still seeking advice on producing
the
> output that matches the other compilers (even if it involves doing a
> translation on the .ll file or additional translation to the produced
> assembly).  We can't fairly compare the code if it doesn't do the
same
> thing.  This bug came up in susan from the mibench test suite at the end of
> setup_brightness_lut where it casts float temp to an unsigned char.  I
> don't think we can include the results in our report if we change the
tests.
>
> Even though the behavior is undefined, it doesn't make sense to me that
a
> function that returns an unsigned char would ever return a 32 bit -1,
> because that value does not fit the functions return type.
>
>
> On Mon, Aug 6, 2012 at 2:03 PM, Eli Friedman <eli.friedman at
gmail.com>wrote:
>
>> On Mon, Aug 6, 2012 at 12:43 PM, ryan baird <ryanrbaird at
gmail.com> wrote:
>> > I am compiling the following code for the MIPS architecture:
>> >
>> > unsigned char trunc(float f) {
>> >         return (unsigned char) f;
>> > }
>> >
>> > and it produces the following assembly (directives removed for
>> convenience:
>> > trunc:
>> >     trunc.w.s    $f0, $f12
>> >     mfc1    $2, $f0
>> >     jr    $ra
>> >     nop
>> >
>> > However, this does not seem to produce the correct output for
negative
>> > numbers. When I run the following code, I get -1 instead of 255
(which
>> is
>> > produced by compiling natively with gcc).
>> > int trunc(float c);
>> > int main() {
>> >   printf("%d\n", trunc(-1.0));
>> > }
>>
>> That code has undefined behavior; see 6.3.1.4p1 in C99
>>
>> -Eli
>>
>
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Richard Smith

2012-Aug-06 21:33 UTC

head link

[LLVMdev] Casting from float to unsigned char - incorrect output?

This should work if your float is between INT_MIN and INT_MAX:

unsigned char trunc(float f) {
  return (unsigned char)(int)f;
}

int main() {
  printf("%d\n", trunc(-1.0));
}

For this, Clang produces:

trunc:
trunc.w.s $f0, $f12
mfc1 $2, $f0
andi $2, $2, 255
jr $ra
nop

On Mon, Aug 6, 2012 at 1:28 PM, ryan baird <ryanrbaird at gmail.com>
wrote:
> I didn't realize the code was undefined, I'll let my professor
know; but
>> for code comparison purposes we're still seeking advice on
producing the
>> output that matches the other compilers (even if it involves doing a
>> translation on the .ll file or additional translation to the produced
>> assembly).  We can't fairly compare the code if it doesn't do
the same
>> thing.  This bug came up in susan from the mibench test suite at the
end of
>> setup_brightness_lut where it casts float temp to an unsigned char.  I
>> don't think we can include the results in our report if we change
the tests.
>>
>> Even though the behavior is undefined, it doesn't make sense to me
that a
>> function that returns an unsigned char would ever return a 32 bit -1,
>> because that value does not fit the functions return type.
>>
>>
>> On Mon, Aug 6, 2012 at 2:03 PM, Eli Friedman <eli.friedman at
gmail.com>wrote:
>>
>>> On Mon, Aug 6, 2012 at 12:43 PM, ryan baird <ryanrbaird at
gmail.com>
>>> wrote:
>>> > I am compiling the following code for the MIPS architecture:
>>> >
>>> > unsigned char trunc(float f) {
>>> >         return (unsigned char) f;
>>> > }
>>> >
>>> > and it produces the following assembly (directives removed for
>>> convenience:
>>> > trunc:
>>> >     trunc.w.s    $f0, $f12
>>> >     mfc1    $2, $f0
>>> >     jr    $ra
>>> >     nop
>>> >
>>> > However, this does not seem to produce the correct output for
negative
>>> > numbers. When I run the following code, I get -1 instead of
255 (which
>>> is
>>> > produced by compiling natively with gcc).
>>> > int trunc(float c);
>>> > int main() {
>>> >   printf("%d\n", trunc(-1.0));
>>> > }
>>>
>>> That code has undefined behavior; see 6.3.1.4p1 in C99
>>>
>>> -Eli
>>>
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
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