[llvm-dev] Why x86_64 divq is not used for 128-bit by 64-bit division?

[Craig Topper via llvm-dev]

divq only produces a 64-bit result. There's way for the compiler to know
you didn't divide a greater than 64-bit number by 1 or some other value
that required a large result.

~Craig

On Thu, Oct 19, 2017 at 1:47 PM, Paweł Bylica via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi there,
>
> Let's have this C code:
>
> unsigned long div(unsigned __int128 n, unsigned long d)
> {
> return n / d;
> }
>
> I would assume that the divq is the perfect match here. But the compiler
> generates the
> code that calls the __udivti3 procedure which performs 128-bit by 128-bit
> division.
>
> Why is divq not used here?
>
> - Paweł
>
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