[llvm-commits] [PATCH] RDRAND intrinsics

[Benjamin Kramer]

On 07.07.2012, at 11:12, Benjamin Kramer wrote:

> 
> On 06.07.2012, at 23:30, Eli Friedman wrote:
> 
>> On Fri, Jul 6, 2012 at 2:03 PM, Benjamin Kramer <benny.kra at gmail.com> wrote:
>>> 
>>> On 06.07.2012, at 22:52, Eli Friedman wrote:
>>> 
>>>> On Fri, Jul 6, 2012 at 1:00 PM, Benjamin Kramer <benny.kra at gmail.com> wrote:
>>>>> The attached patch adds support for intel's rdrand intrinsics to LLVM.
>>>>> 
>>>>> The tricky part is that the intrinsics have to return two values (the random value and the carry flag). This is solved with a pointer argument and a cmov to materialize CF, which matches GCC's output. This approach requires some custom lowering code, but I don't see a way to do it with tablegenerated patterns only.
>>>> 
>>>> It would be much more friendly to the optimizer to use multiple return
>>>> values... but it might not be worth the hassle, considering that
>>>> rdrand is relatively slow.
>>> 
>>> I tried this first as it also looks cleaner, but I couldn't find a way to smuggle it past legalization into the target specific lowering.
>> 
>> X86TargetLowering::ReplaceNodeResults?
> 
> Doesn't work because TLI.getOperationAction bails out early when it sees an extended type. The legalizer then tries to expand the node and aborts.

Apparently I made some mistake yesterday, it *does* work without applying any tricks as long as all return values are legal individually. I attached an updated patch which uses the following intrinsic signature, allowing the store to stay in IR.

declare { i16, i32 } @llvm.x86.rdrand.16()

Here i16 is the random value and i32 is the boolean indicating whether it is considered valid. I also attached the clang part of this patch and reverified that the code matches what ICC and GCC emit.

- Ben

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