[llvm-dev] How to recover function parameter Value* when -O2 active

[Johannes Doerfert via llvm-dev]

If you want to access the `llvm::AllocaInst` generated by Clang for
the argument you should run your pass early in the pipeline. If you
just want to see the value passed to `bar`, take the `llvm::Argument`
and pass it to your profiling function. If you need indirection for
some reason, create a new `llvm::AllocaInst`, store the `llvm::Argument`
you are interested in in it, and pass the `llvm::AllocaInst` to your
function.

I hope this helps.

~ Johannes


On 2/23/21 3:19 PM, K Jelesnianski via llvm-dev wrote:
> Dear LLVM mailing list,
>
> I am currently having difficulty with figuring out how to get a function
> parameter Value* when I am compiling with -O2. It seems -O2 performs an
> optimization that optimizes out the local variable formed for the function
> parameter.
> I have a pass where I am inserting new function calls, and need to pass the
> function parameters variable address. With -O0 this issue does not come up
> as each function parameter when using -O0 gets its own AllocaInst and I can
> leverage that Value* to pass to my new function call. I have tried to
> leverage llvm.dbg information but it seems LLVM is not inserting
> llvm.dbg.addr for me to use.
>
> Below is the code example I am working with:
> /*  example.c */
> int fd_in;
> char* ut_addr;
>
> __attribute__((noinline)) void bar (int b){
>         my_pass_function( &b );  // NEW (what I want to be done by my pass)
>         ut_addr = (char*) mmap( 0, b, my_prot, MAP_PRIVATE, fd_in, 0);
> }
>
> __attribute__((noinline)) void foo (int f){
>          bar( f );
> }
>
> int main(int argc, char** argv){
>          int m;
>          struct stat statbuf;
>          char *input;
>
>          if( (fd_in = open ( input, O_RDONLY)) < 0){        /* open the
> input file */
>                  return 0;
>          }
>          if( fstat( fd_in , &statbuf ) < 0 ){         /* find size of input
> file */
>                  return 0;
>          }
>
>          m = statbuf.st_size;
>          foo( m );
>          return 0;
> }
> -----------------------------------------------------------------
> The original IR I get for bar() is below, as you can see the function
> parameter is the "raw" value "i32 %b" passed into the function and not a
> variable that I can leverage, get its Value* . Hence when I run my newly
> instrumented code, I get a segmentation fault.
>
> define void @bar(i32 %b) local_unnamed_addr #0 !dbg !35 {
> entry:
>    call void @llvm.dbg.value(metadata i32 %b, metadata !39, metadata
> !DIExpression()), !dbg !44
>    call void @llvm.dbg.value(metadata i64 1000, metadata !40, metadata
> !DIExpression()), !dbg !44
>    call void @llvm.dbg.value(metadata i32 1, metadata !43, metadata
> !DIExpression()), !dbg !44
>    %conv = sext i32 %b to i64, !dbg !45
>    %0 = load i32, i32* @fd_in, align 4, !dbg !46, !tbaa !47
>    %call = tail call i8* @mmap(i8* null, i64 %conv, i32 1, i32 2, i32 %0,
> i64 0) #7, !dbg !51
>    store i8* %call, i8** @ut_addr, align 8, !dbg !52, !tbaa !53
>    ret void, !dbg !55
> }
>
>
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