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

[K Jelesnianski via llvm-dev]

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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