[llvm-dev] Preserving Call to Intrinsic function

Pierre-Andre Saulais via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 27 09:05:21 PST 2017

Hi Kumail,

There is a flag that does what you are looking for (see Intrinsics.td
for definitions):

// IntrNoduplicate - Calls to this intrinsic cannot be duplicated.
// Parallels the noduplicate attribute on LLVM IR functions.
def IntrNoDuplicate : IntrinsicProperty;

Another thing to check is whether your intrinsic is marked as having
side-effects or not. Intrinsics with no side effects may be optimized by
LLVM. If you inspect the DAG using llc, an intrinsic with side-effects
should have a chain (blue dashed edge) going to and coming from its node:

llc -O1 foo.ll -view-isel-dags

If it doesn't, you can mark it as having side-effects using one of the
following attributes:

// IntrReadMem - This intrinsic only reads from memory. It does not write to
// memory and has no other side effects. Therefore, it cannot be moved
// potentially aliasing stores. However, it can be reordered otherwise
and can
// be deleted if dead.
def IntrReadMem : IntrinsicProperty;

// IntrWriteMem - This intrinsic only writes to memory, but does not
read from
// memory, and has no other side effects. This means dead stores before
// to this intrinsics may be removed.
def IntrWriteMem : IntrinsicProperty;

By default intrinsics returning void are assumed to have side effects.


On 27/01/17 16:33, Kumail Ahmed via llvm-dev wrote:
> Hello everyone,
> Consider we have this following set of code:
> int foo() {
> int a,b;
> a = __builtin_XX(0x11);
> b = __builtin_XX(0x11);
> return a+b;
> }
> The problem currently is that LLVM eliminated the second call and
> copied the result from the first call into a new set of registers. Is
> there is a way to force LLVM to generate two explicit calls to a
> builtin function. The builtin takes in an integer type, and also
> returns back an integer type:
>   def int_XX : GCCBuiltin<"__builtin_XX">, Intrinsic<[llvm_i32_ty],
> [llvm_i32_ty]>;
> Is there some flag that I'm missing? Your help will be really appreciated.
> Best Regards,
> Kumail Ahmed
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Pierre-Andre Saulais
Principal Software Engineer, Compilers
Codeplay Software Ltd
Level C Argyle House, 3 Lady Lawson Street, Edinburgh, United Kingdom, EH3 9DR
Tel: +44 (0)131 466 0503
Website: http://www.codeplay.com
Twitter: https://twitter.com/codeplaysoft

This email and any attachments may contain confidential and /or privileged information and is for use by the addressee only. If you are not the intended recipient, please notify Codeplay Software Ltd immediately and delete the message from your computer. You may not copy or forward it, or use or disclose its contents to any other person. Any views or other information in this message which do not relate to our business are not authorized by Codeplay software Ltd, nor does this message form part of any contract unless so stated.
As internet communications are capable of data corruption Codeplay Software Ltd does not accept any responsibility for any changes made to this message after it was sent. Please note that Codeplay Software Ltd does not accept any liability or responsibility for viruses and it is your responsibility to scan any attachments.
Company registered in England and Wales, number: 04567874
Registered office: Regent House, 316 Beulah Hill, London, United Kingdom, SE19 3HF

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170127/3ad06e64/attachment.html>

More information about the llvm-dev mailing list