[LLVMdev] Overflow trap
clattner at apple.com
Mon Aug 9 17:45:41 PDT 2010
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.
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