[llvm] r206732 - Implement builtins for safe division: safe.sdiv.iN, safe.udiv.iN, safe.srem.iN,

Andrew Trick atrick at apple.com
Tue Apr 22 17:25:17 PDT 2014

On Apr 22, 2014, at 8:19 AM, Eric Christopher <echristo at gmail.com> wrote:

>> The goal here is to complete the implementation of LLVM’s existing arithmetic with overflow intrinsics. It was originally llvm.sdiv.with.overflow. I objected to that name because it is technically not checking overflow, so I thought that name would be misleading. To me overflow on division means taking a numerator composed of two 64-bit operands and expecting a single 64-bit result (as with x86).
> This is particularly unclear.

The name of the intrinsic is not something we need to quibble over, but I'll try to clarify:

IDIV: Perform signed division of RDX:RAX by the contents of a 64-bit register or memory location and store the quotient in RAX and the remainder in RDX. 

So if, for example, the divisor is 1 and RDX is not the sign bit of RAW, we hit what I would call overflow.

The LLVM division intrinsics don't deal with this issue, and I don’t want to confuse people into thinking they do. Rather, they implement the semantics of integer division that I think we want at the IR level  (where each operand is the same size as the result).

div(x, 0) == 0
sdiv(min<T>, -1) == min<T>

(this happens to match arm64)

The intrinsics return a boolean to indicate that "overflow" occured, just like all the other llvm.arith.with.overflow intrinsics.

I think an llvm-dev posting will also summarize this.

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