[llvm-dev] Preserving Call to Intrinsic function

Kumail Ahmed via llvm-dev llvm-dev at lists.llvm.org
Fri Jan 27 09:50:07 PST 2017

Hi Pierre,

Thank you for your swift response!

I've tried almost everything that you mentioned, but it seems like LLVM
simply doesn't care about the flags I pass to it.

Best regards,
Kumail Ahmed

On Fri, Jan 27, 2017 at 6:05 PM, Pierre-Andre Saulais <
pierre-andre at codeplay.com> wrote:

> 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
> across
> // 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
> calls
> // to this intrinsics may be removed.
> def IntrWriteMem : IntrinsicProperty;
> By default intrinsics returning void are assumed to have side effects.
> Pierre-Andre
> 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 listllvm-dev at lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> --
> Pierre-Andre Saulais
> Principal Software Engineer, Compilers
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Best Regards,
Kumail Ahmed
M.Sc. Student
TU Kaiserslautern
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