[LLVMdev] 8-bit DIV IR irregularities

[Eli Friedman]

On Wed, Jun 27, 2012 at 5:22 PM, Nowicki, Tyler <tyler.nowicki at intel.com> wrote:
> 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?

The IR instruction has undefined behavior on overflow.  This has
nothing to do with legalization.

> 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.

There is no way to write an 8-bit divide in C; it's a 32-bit divide
where each operand happens to be a sign extension from an 8-bit type.

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

For your div8 testcase, instcombine transforms from a "udiv i32" to a
"udiv i8".  instcombine isn't allowed to do that for "sdiv i32"
because it potentially introduces undefined behavior.

-Eli