llvm dev - Mar 2008 - [LLVMdev] Checked arithmetic

Hi Chris,
> Why not define an "add with overflow" intrinsic that returns its
value and
> overflow bit as an i1?
what's the point?  We have this today with apint codegen (if you turn on
LegalizeTypes).  For example, this function

define i1 @cc(i32 %x, i32 %y) {
  %xx = zext i32 %x to i33
  %yy = zext i32 %y to i33
  %s = add i33 %xx, %yy
  %tmp = lshr i33 %s, 32
  %b = trunc i33 %tmp to i1
  ret i1 %b
}

codegens (on x86-32) to

cc:
        xorl    %eax, %eax
        movl    4(%esp), %ecx
        addl    8(%esp), %ecx
        adcl    $0, %eax
        andl    $1, %eax
        ret

which uses the condition code as desired.  Pity about the
redundant andl $1, %eax!

Ciao,

Duncan.

On Wed, 26 Mar 2008, Duncan Sands wrote:
> Hi Chris,
>
>> Why not define an "add with overflow" intrinsic that returns
its value and
>> overflow bit as an i1?
>
> what's the point?  We have this today with apint codegen (if you turn
on
> LegalizeTypes).  For example, this function
The desired code is something like:

foo:
    addl %eax, %ecx
    jo overflow_happened
    use(%ecx)

etc.

-Chris
> define i1 @cc(i32 %x, i32 %y) {
>  %xx = zext i32 %x to i33
>  %yy = zext i32 %y to i33
>  %s = add i33 %xx, %yy
>  %tmp = lshr i33 %s, 32
>  %b = trunc i33 %tmp to i1
>  ret i1 %b
> }
>
> codegens (on x86-32) to
>
> cc:
>        xorl    %eax, %eax
>        movl    4(%esp), %ecx
>        addl    8(%esp), %ecx
>        adcl    $0, %eax
>        andl    $1, %eax
>        ret
>
> which uses the condition code as desired.  Pity about the
> redundant andl $1, %eax!
>
> Ciao,
>
> Duncan.
>
-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

Hi Chris,
> > what's the point?  We have this today with apint codegen (if you
turn on
> > LegalizeTypes).  For example, this function
> 
> The desired code is something like:
> 
> foo:
>     addl %eax, %ecx
>     jo overflow_happened
>     use(%ecx)
how's an intrinsic going to help with this?  Also, is there any chance
of codegen being capable one day of combining carry arithmetic coming
from apint and the appropriate conditional branch into a "jo"?

Ciao,

Duncan.