llvm dev - Jun 2016 - How can I make llvm intrinsic functions declarations survive from optimizations.

Dear LLVM community.

I have a function pass. This pass will insert a llvm intrinsic function 
prototype (llvm.memset.p0i8.i32) to the module at the doInitialize 
stage. Then at runOnFunction Stage, this pass will find the insert 
points and insert calls of this intrinsic function. However, when the 
-O2 flag is turned on, the strip unused function optimization will 
automatically delete this intrinsic function because there's no use of 
this function found. (Strip unused function happens before runOnFunction 
of my pass).

I used to solve this kind of problems on non-intrinsic functions by 
inserting a record to llvm.compiler.used. However, after I append this 
intrinsic function to llvm.compiler.used global, I got a fatal error

Invalid user of intrinsic instruction!
i8* bitcast (void (i8*, i8, i32, i32, i1)* @llvm.memset.p0i8.i32 to i8*)
fatal error: error in backend: Broken module found, compilation aborted!

I noticed that, only user of direct call/invokes are allowed for 
intrinsic functions. So is it impossible to solve this problem by 
inserting intrinsic function prototype?

Could you please give me some ideas on this , to make these intrinsic 
functions prototypes survive from llvm optimizations?

Thank you.

Best regards,
Zhengyang.

Hi Zhengyang,

Do you mind sharing _why_ you need the intrinsic declarations to stay
around?  It is possible that there is a better way of solving your
problem.

Thanks!
-- Sanjoy

Dear Sanjoy Das and community.


I was tried to fix a bug in the pass InitAllocas from SAFECode. This is a
function pass and will insert a prototype of 'llvm.memset.p0i8.i32' in
the module at doInitialize() stage of the pass. But, this prototype will be
eliminated by strip unused function optimization since there is no call on this
function after doInitialization(). Therefore there will be no prototype at the
runOnFunction() stage, and this will cause a fail. Previously I solve this kind
of bug by inserting the prototype of the function to llvm.compiler.used. In this
way, the compiler will not leave the prototype alone. This time, the situation
is a bit complicated, inserting the intrinsic function to llvm.compiler.used
will cause a fail because only users of direct call/invokes are allowed on
intrinsic functions.


Fortunately, after some discussion with my GSoC mentor, Prof. Criswell, we chose
to rewrite the pass to a module pass, this solves this issue perfectly. Thanks
for your patience.


Best regards,

Zhengyang.

________________________________
From: Sanjoy Das <sanjoy at playingwithpointers.com>
Sent: Thursday, June 23, 2016 3:44:09 PM
To: Zhengyang Liu
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] How can I make llvm intrinsic functions declarations
survive from optimizations.

Hi Zhengyang,

Do you mind sharing _why_ you need the intrinsic declarations to stay
around?  It is possible that there is a better way of solving your
problem.

Thanks!
-- Sanjoy
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