[all-commits] [llvm/llvm-project] 6b55a9: [DebugInfo] Emit DW_OP_implicit_value for Floating...

[Sourabh Singh Tomar via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 6b55a95898e98664164caae4aba7c5e24fd1a05e
      https://github.com/llvm/llvm-project/commit/6b55a95898e98664164caae4aba7c5e24fd1a05e
  Author: Sourabh Singh Tomar <SourabhSingh.Tomar at amd.com>
  Date:   2020-07-23 (Thu, 23 Jul 2020)

  Changed paths:
    M llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp
    M llvm/lib/CodeGen/AsmPrinter/DwarfExpression.cpp
    M llvm/lib/CodeGen/AsmPrinter/DwarfExpression.h
    M llvm/test/DebugInfo/X86/float_const_loclist.ll
    A llvm/test/DebugInfo/X86/implicit_value-double.ll
    A llvm/test/DebugInfo/X86/implicit_value-float.ll
    A llvm/test/DebugInfo/X86/implicit_value-ld.ll

  Log Message:
  -----------
  [DebugInfo] Emit DW_OP_implicit_value for Floating point constants

Summary:
llvm is missing support for DW_OP_implicit_value operation.
DW_OP_implicit_value op is indispensable for cases such as
optimized out long double variables.

For intro refer: DWARFv5 Spec Pg: 40 2.6.1.1.4 Implicit Location Descriptions

Consider the following example:
```
int main() {
        long double ld = 3.14;
        printf("dummy\n");
        ld *= ld;
        return 0;
}
```
when compiled with tunk `clang` as
`clang test.c -g -O1` produces following location description
of variable `ld`:
```
DW_AT_location        (0x00000000:
                     [0x0000000000201691, 0x000000000020169b): DW_OP_constu 0xc8f5c28f5c28f800, DW_OP_stack_value, DW_OP_piece 0x8, DW_OP_constu 0x4000, DW_OP_stack_value, DW_OP_bit_piece 0x10 0x40, DW_OP_stack_value)
                  DW_AT_name    ("ld")
```
Here one may notice that this representation is incorrect(DWARF4
stack could only hold integers(and only up to the size of address)).
Here the variable size itself is `128` bit.
GDB and LLDB confirms this:
```
(gdb) p ld
$1 = <invalid float value>
(lldb) frame variable ld
(long double) ld = <extracting data from value failed>
```

GCC represents/uses DW_OP_implicit_value in these sort of situations.
Based on the discussion with Jakub Jelinek regarding GCC's motivation
for using this, I concluded that DW_OP_implicit_value is most appropriate
in this case.

Link: https://gcc.gnu.org/pipermail/gcc/2020-July/233057.html

GDB seems happy after this patch:(LLDB doesn't have support
for DW_OP_implicit_value)
```
(gdb) p ld
p ld
$1 = 3.14000000000000012434
```

Reviewed By: aprantl

Differential Revision: https://reviews.llvm.org/D83560