[llvm-dev] Seeking clarification and way forward on limited scope variables.
David Blaikie via llvm-dev
llvm-dev at lists.llvm.org
Wed Apr 15 13:15:34 PDT 2020
On Wed, Apr 15, 2020 at 11:52 AM Vedant Kumar <vedant_kumar at apple.com>
wrote:
> Hi Sourabh,
>
> Thanks for raising this issue. To answer your question, (afaik) there
> isn’t anyone working on DW_AT_start_scope support in tree. We’re looking
> for a solution to this problem for Swift debugging, where it's important
> not to make a debug location for a variable available until its
> (guaranteed) initialization is complete.
>
> If at all possible, I’d /much/ rather we use the existing location list
> machinery to solve this problem. Fundamentally, we’re looking for a way to
> express when a location for a variable becomes available, and location
> lists give us that already.
>
> To test this out, I took the IR for your test case, replaced all calls to
> “dbg.declare(…)” with “dbg.value(…, DW_OP_deref)”, and compiled with `-g
> -O0 -mllvm -fast-isel=false` (apparently FastISel doesn’t know what to do
> with frame-index dbg.values, but this is simple to fix). This seemed to
> work pretty well, and the DWARF looked legit:
>
> ```
> 0x0000005f: DW_TAG_variable
> DW_AT_location (DW_OP_fbreg -20)
> DW_AT_name ("Local")
>
> 0x0000006d: DW_TAG_lexical_block
> DW_AT_low_pc (0x0000000100000f4f)
> DW_AT_high_pc (0x0000000100000f84)
>
> 0x0000007a: DW_TAG_variable
> DW_AT_location (0x00000000
> [0x0000000100000f63, 0x0000000100000f8c):
> DW_OP_breg6 RBP-24)
> DW_AT_name ("Local”)
> ```
>
> I did find one lldb bug where it didn’t know to look up a variable in the
> parent scope when stopped at your second printf() call (line 6). But that's
> a general bug: we’d have to fix it even if we used DW_AT_start_scope.
>
> The upshot of sticking to location lists is that fixing any bugs we find
> improves optimized debugging workflows. And Swift -Onone debugging
> workflows as well, since Swift runs certain mandatory optimizations which
> can make the DWARF at -Onone fairly complex.
>
I think these are different problems, and that location lists can't be used
to express when the scope of a variable starts - Sourabh's example at the
end shows why: that the debugger wouldn't search outside the scope, it
would instead (correctly) report the variable's value as unavailable.
"I did find one lldb bug where it didn’t know to look up a variable in the
parent scope when stopped at your second printf() call (line 6)."
I don't think that's not a bug, though/that change would be incorrect in
general (& no way to differentiate the correct and incorrect places without
scope_start), for example:
void f1();
int i;
void f2() {
int i = 3;
f1();
int j;
i = j;
f1();
}
If you search into outer scopes whenever a location list doesn't cover an
address - then at the second call to 'f1' the debugger would interpret 'i'
as referring to global 'i' when it should be referring to local 'i' just
with no known value. Printing global 'i' could be quite
confusing/misleading.
> best,
> vedant
>
> On Apr 15, 2020, at 9:53 AM, David Blaikie via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> As always, concerned about the size growth in object files this might
> produce - though looks like the DWARF spec avoids the worst of this in
> unoptimized code by using an offset relative to the start of the enclosing
> scope, so it doesn't require a relocation in that case.
>
> I have no idea what the LLVM DWARF representation for this would look like
> - short of making even more fine-grained scopes in the DILexicalScope
> hierarchy, which sounds really expensive from a memory perspective. That's
> really where I worry that the cost to this feature might outweigh the
> benefit (& might be why no one's done this in the past) - but data should
> tell us that. As much as in-tree development is preferred, this might be
> the sort of thing worth prototyping out of tree first to see if it can be
> made viable before adding the complexity to the LLVM project proper - but
> I'm open to ideas/suggestions.
>
> On Wed, Apr 15, 2020 at 3:15 AM Sourabh Singh Tomar <sourav0311 at gmail.com>
> wrote:
>
>> Hello Everyone,
>>
>>
>> I need to have your thoughts on this.
>>
>>
>> Consider the following test case --
>> -------------------------------------------
>> 1 int main(int Argc, char **Argv) {
>> 2 int Local = 6;
>> 3 printf("%d\n",Local);
>> 4
>> 5 {
>> 6 printf("%d\n",Local);
>> 7 int Local = 7;
>> 8 printf("%d\n",Local);
>> 9 }
>> 10
>> 11 return 0;
>> 12 }
>> --------------------------------------------
>> When compiled in debug mode with compilers including (trunk gcc and trunk
>> clang) and debugging with GDB at Line No.6, the following behavior is
>> observed
>> Breakpoint 1, main (Argc=1, Argv=0x7fffffffe458) at MainScope.c:6
>> 6 printf("%d\n",Local);
>> (gdb) print Local
>> $1 = 2102704 -- some Garbage value,
>> (gdb) info addr Local
>> Symbol "Local" is a variable at frame base reg $rbp offset 0+-24. -- *This
>> is location of *Local* declared inside scope, but as you may notice that
>> the variable being referred here is from the outer scope.*
>>
>>
>> This problem persists with both GDB and LLDB. Since we have entered the
>> Lexical Scope and when we try to print value of *Local*, it will look into
>> the *current scope* and fetch the value if the variable exists in scope(in
>> case variable doesn't exist, GDB searches for it in the outer scope).
>>
>>
>> This is regardless of whether the variable has actually came into
>> scope(or actually defined) at Line No. 7. Since DWARF already defined the
>> location(on stack) which will be valid for the lifetime of the variable,
>> contrary to when the variable is actually defined(or allocated) which is in
>> this case Line No. 7.
>> ---------------------------------------------
>> 0x0000006d: DW_TAG_lexical_block
>> DW_AT_low_pc (0x00000000002016d1)
>> DW_AT_high_pc (0x000000000020170b)
>> 0x0000007a: DW_TAG_variable
>> DW_AT_location (DW_OP_fbreg -24)
>> DW_AT_name ("Local")
>> DW_AT_decl_file ("MainScope.c")
>> DW_AT_decl_line (7)
>> DW_AT_type (0x0000008a "int")
>> ----------------------------------------------
>>
>>
>> The DWARF specification provides the DW_AT_start_scope attribute to deal
>> with this issue (Sec 3.9 Declarations with Reduced Scope DWARFv5). This
>> attribute aims at limiting the scope of variables within the lexical scope
>> in which it is defined to from where it has been declared/ defined.
>>
>>
>> In order to fix this issue, we want to modify llvm so that
>> DW_AT_start_scope is emitted for the variable in the inner block (in the
>> above example). This limits the scope of the inner block variable to start
>> from the point of its declaration.
>>
>>
>> For POC, we inserted DW_AT_start_scope in this inner *Local* variable,
>> resultant dwarf after this.
>> -----------------------------
>> 0x0000006d: DW_TAG_lexical_block
>> DW_AT_low_pc (0x00000000002016d1)
>> DW_AT_high_pc (0x000000000020170b)
>> 0x0000007a: DW_TAG_variable
>> * DW_AT_start_scope (0x17) -- restricted within a
>> subset(starting from the point of definition(specified as an offset)) of
>> entire ranges covered by Lex Block.*
>> DW_AT_location (DW_OP_fbreg -24)
>> DW_AT_name ("Local")
>> DW_AT_decl_file ("MainScope.c")
>> DW_AT_decl_line (7)
>> DW_AT_type (0x00000092 "int")
>> ----------------------------
>>
>>
>>
>>
>> We also modified ‘gdb’ to interpret DW_AT_start_scope so that the scope
>> of the variable is limited from the PC where the value of DW_AT_start_scope
>> is. If the debugger is stopped at a point within the same lexical block but
>> at a PC before DW_AT_start_scope, then gdb follows the normal search
>> mechanism of searching in consecutive super blocks till it gets a match or
>> it reaches the global block. After the modification, GDB is able to
>> correctly show the value *6* in our example.
>>
>>
>>
>>
>> After incorporating changes --
>> Breakpoint 1, main (Argc=1, Argv=0x7fffffffe458) at MainScope.c:6
>> 6 printf("%d\n",Local);
>> (gdb) print Local
>> * $1 = 6 --- Value retrieved from outer scope*
>> (gdb) info addr Local
>>
>> Symbol "Local" is a variable at frame base reg $rbp offset 0+-20.
>>
>>
>> Could you guys please let us know your thoughts or suggestions on this?
>> Was/ Is there is an existing effort already going on to deal with this
>> problem?
>>
>>
>> Even though location lists can be used to deal with this scenario, in my
>> experience, location lists are emitted at higher optimization levels, and
>> with the usage of location lists in this example, gdb prints out <optimized
>> out> (as expected) if it is stopped at a PC in the same lexical block but
>> before the point of declaration of the local variable.
>>
>> Thank You,
>> Sourabh Singh Tomar.
>>
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