[llvm-bugs] [Bug 39710] New: Emission of DW_TAG_formal_parameter location is inaccurate on Linux (with -O3)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=39710

            Bug ID: 39710
           Summary: Emission of DW_TAG_formal_parameter location is
                    inaccurate on Linux (with -O3)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: DebugInfo
          Assignee: unassignedbugs at nondot.org
          Reporter: davide at freebsd.org
                CC: aprantl at apple.com, friss at apple.com,
                    jdevlieghere at apple.com, keith.walker at arm.com,
                    llvm-bugs at lists.llvm.org, rnk at google.com,
                    vsk at apple.com

There has been a lot of talking about about unrecoverable function arguments
for optimized builds, so I decided to give it a try to see if there's something
that could be done to improve.

Take the following test case:

__attribute__((noinline))
int fn1 (long int x, long int y, long int z) {
        int l = x * 2;
        int q = y * z;
        return l + q;
}

__attribute__((noinline)) long int
fn2 (long int a, long int b, long int c)
{
  long int q = 2 * a;
  return fn1 (5, 6, 7);
}

int main(void) {
        return fn2(14, 23, 34);
}

Build with -O3 -g. Set a breakpoint on fn1, and try to print the arguments
(e.g. through `frame var`).

You'll realize that you can recover them on a recent MacOS (Mojave), but you
can't on ubuntu 18.10.

I started looking a little bit into why this was the case, to immediately find
out the DWARF emission for the formal parameter differs on the two platforms.

Namely:

MacOS:

0x00000043:     DW_TAG_formal_parameter
                  DW_AT_location        (0x00000000
                     [0x0000000100000f70,  0x0000000100000f74): DW_OP_reg5 RDI
                     [0x0000000100000f74,  0x0000000100000f7c): DW_OP_reg5 RDI)
                  DW_AT_name    ("x")
                  DW_AT_decl_file      
("/Users/davide/llvm-monorepo/llvm-mono/build/bin/optout.c")
                  DW_AT_decl_line       (2)
                  DW_AT_type    (0x00000104 "long int")

Linux:
0x00000043:     DW_TAG_formal_parameter
                  DW_AT_location        (0x00000000)
                  DW_AT_name    ("x")
                  DW_AT_decl_file       ("/home/davide/optimized_code/./opt.c")
                  DW_AT_decl_line       (2)
                  DW_AT_type    (0x000000f1 "long int")


While the former contains a location list pointing at the register where the
debugger should retrieve the argument, the second one contains an empty
location list.

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