llvm dev - Jun 2012 - [LLVMdev] 8-bit DIV IR irregularities

Hi,

I noticed that when dividing with signed 8-bit values the IR uses a 32-bit
signed divide, however, when unsigned 8-bit values are used the IR uses an 8-bit
unsigned divide. Why not use a 8-bit signed divide when using 8-bit signed
values?

Here is the C code and IR:

char idiv8(char a, char b)
{
  char c = a / b;
  return c;
}

define signext i8 @idiv8(i8 signext %a, i8 signext %b) nounwind readnone {
entry:
  %conv = sext i8 %a to i32
  %conv1 = sext i8 %b to i32
  %div = sdiv i32 %conv, %conv1
  %conv2 = trunc i32 %div to i8
  ret i8 %conv2
}

unsigned char div8(unsigned char a, unsigned char b)
{
  unsigned char c = a / b;
  return c;
}

define zeroext i8 @div8(i8 zeroext %a, i8 zeroext %b) nounwind readnone {
entry:
  %div3 = udiv i8 %a, %b
  ret i8 %div3
}

I noticed the same behavior in O3. The command line arguments I'm using for
clang are: -O2 -emit-llvm -S.

Tyler Nowicki
Intel

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On Wed, Jun 27, 2012 at 4:02 PM, Nowicki, Tyler <tyler.nowicki at
intel.com> wrote:
> Hi,
>
>
>
> I noticed that when dividing with signed 8-bit values the IR uses a 32-bit
> signed divide, however, when unsigned 8-bit values are used the IR uses an
> 8-bit unsigned divide. Why not use a 8-bit signed divide when using 8-bit
> signed values?
"sdiv i8 -128, -1" has undefined behavior; "sdiv i32 -128,
-1" is well-defined.

-Eli

I understand, but this sounds like legalization. Does every architecture trigger
an overflow exception, as opposed to setting a bit? Perhaps it makes more sense
to do this in the backends that trigger an overflow exception?

I'm working on a modification for DIV right now in the x86 backend for Intel
Atom that will improve performance, however because the *actual* operation has
been replaced with a 32-bit operation it becomes much more difficult to detect
when a real 32-bit divide is happening.

If someone knows where the 8-bit DIV is being handled in the IR I could look
into this change?

Tyler

-----Original Message-----
From: Eli Friedman [mailto:eli.friedman at gmail.com] 
Sent: Wednesday, June 27, 2012 19:07 PM
To: Nowicki, Tyler
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] 8-bit DIV IR irregularities

On Wed, Jun 27, 2012 at 4:02 PM, Nowicki, Tyler <tyler.nowicki at
intel.com> wrote:
> Hi,
>
>
>
> I noticed that when dividing with signed 8-bit values the IR uses a 
> 32-bit signed divide, however, when unsigned 8-bit values are used the 
> IR uses an 8-bit unsigned divide. Why not use a 8-bit signed divide 
> when using 8-bit signed values?
"sdiv i8 -128, -1" has undefined behavior; "sdiv i32 -128,
-1" is well-defined.

-Eli