[LLVMdev] Overflow trap

[Chris Lattner]

After chatting on IRC, Peter wants a very specific interrupt (int4 on x86).  I suggested he add a new llvm.x86.int(i32) intrinsic, and use the existing branch on llvm.sadd.with.overflow intrinsic.  The x86 backend can then turn jo+int4 into into when reasonable.

-Chris

On Aug 9, 2010, at 5:45 PM, Chris Lattner wrote:

> 
> On Aug 9, 2010, at 10:44 AM, Peter S. Housel wrote:
> 
>> Several instruction set architectures include arithmetic operations that can trap on overflow, or support this feature with a separate trap-on-overflow-flag instruction (such as the x86 INTO instruction).
>> 
>> 
>> I am adding a back-end to the Open Dylan compiler to generate LLVM IR. The original back-end, which generates x86 machine code, makes use of the INTO instruction, and the runtime turns the resulting INT 4 into a language-level exception. In order to support this not-uncommon requirement using LLVM, I see three alternatives:
>> 
>> 1. Add llvm.sadd.with.overflow.trap.*, … intrinsics corresponding to the current llvm.sadd.with.overflow.*, … intrinsics to LLVM and the current code generators.
>> 
>> 2. Add a single llvm.trap.overflow intrinsic to LLVM and the current code generators, and ensure that the code generators can generate the proper arithmetic-with-overflow-trap or trap-on-overflow-flag instruction when it is executed conditionally based on the output of one of the llvm.sadd.with.overflow.* etc. intrinsics.
>> 
>> 3. Don’t try to solve this problem for LLVM in general but use inline assembly for these few operations in my back-end.
>> 
>> Which alternative should I pursue? The third is obviously the easiest for me, but other LLVM users would benefit from the other two.
>> 
>> 
> 
> I don't understand... why can't you use a branch on a llvm.sadd.with.overflow that goes to an llvm.trap?  This is what clang does with -ftrapv.
> 
> -Chris

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