commit 79a58faf7c54eca5fabf4f2e930b046301862c07
parent 6cb2eecdb1242424830f247da2c07c33942ed99a
Author: Luís Ferreira <contact@lsferreira.net>
Date: Thu, 11 Nov 2021 16:07:08 +0000
posts: Add 'SAOC LLDB D integration: 8th Weekly Update'
Signed-off-by: Luís Ferreira <contact@lsferreira.net>
Diffstat:
2 files changed, 139 insertions(+), 1 deletion(-)
diff --git a/content/posts/d-saoc-2021-07.md b/content/posts/d-saoc-2021-07.md
@@ -93,4 +93,5 @@ and my productive is limited. I hope next week I can do a bit more stuff and
hopefully fix most of the issues I have in mind to homogenize the DWARF
generation.
-Read about the [previous week](../d-saoc-2021-06/).
+Read about the [previous week](../d-saoc-2021-06/) and the [next
+week](../d-saoc-2021-08/).
diff --git a/content/posts/d-saoc-2021-08.md b/content/posts/d-saoc-2021-08.md
@@ -0,0 +1,137 @@
+---
+title: 'SAOC LLDB D integration: 8th Weekly Update'
+date: '2021-11-11T15:00:00+01:00'
+tags: ['saoc', 'saoc2021', 'dlang', 'llvm', 'lldb', 'debug', 'debugging', 'dwarf']
+description: "This post describes what I've done on the 8th week of the
+Symmetry Autumn of Code 2021, including follow up updates on the LLVM patches,
+some bug hunting, various fixes, LLVM ustream support of immutable type
+qualifier and D calling convention issue."
+---
+
+Hi D community!
+
+I'm here again, to describe what I've done during the eighth week of Symmetry
+Autumn of Code. Sorry for being a bit late.
+
+## LLVM upstream follow up update
+
+The following patches got merged:
+- https://reviews.llvm.org/D111414
+- https://reviews.llvm.org/D111432
+- https://reviews.llvm.org/D110578
+
+This includes the most important patch of the Milestone 1, which is initial
+support for D demangling. After a conversation with Chris Lattner (LLVM lead
+developer and creator) I managed to get acceptance on merging that change,
+leaving relicensing issues aside from my task list. This also means that
+minimal support for D in LLDB is now on the official tree.
+
+Right after the merge, I submitted a patch to Google OSS Fuzz, to start fuzzing
+D demangler: https://github.com/google/oss-fuzz/pull/6811 . Some more patches
+are maybe required, since the last docker is really out-of-date and is using
+deprecated features of the LLVM build system.
+
+## Fixed issues and bug hunting
+
+### Types with wrong exported names in DWARF
+
+As reported in the previous week, I pushed a fix for [issue
+22469](https://issues.dlang.org/show_bug.cgi?id=22469) this week, in which you
+can find [here](https://github.com/dlang/dmd/pull/13274). I also fixed a nit
+specification issue, https://github.com/dlang/dlang.org/pull/3119 .
+
+So TL;DR, now debug info with DMD should report the following type names
+instead:
+
+```
+wchar_t -> wchar
+long double -> real
+_Bool -> bool
+long long -> long
+uint long long -> ulong
+imaginary float -> ifloat
+imaginary double -> idouble
+imaginary long double -> ireal
+complex float -> cfloat
+complex double -> cdouble
+complex long double -> creal
+```
+
+### Delegates debug info now conform with the specification on LDC
+
+This trivial change on LDC was required in order to provide the current member
+name, according to the specification:
+https://github.com/ldc-developers/ldc/pull/3866 .
+
+### LDC should generate type qualifier tags
+
+I though that LDC issues was trivial to fix, but some required upstream
+support. This particular issue, is not a blocker, since only immutable tag is
+missing on the upstream. So, I made a collection of patches to fix that there:
+- https://reviews.llvm.org/D113632
+- https://reviews.llvm.org/D113633
+- https://reviews.llvm.org/D113634
+
+You can follow up this issue
+[here](https://github.com/ldc-developers/ldc/issues/3867) and
+[here](https://bugs.llvm.org/show_bug.cgi?id=52471).
+
+### LDC should generate column DECL attributes on debug info
+
+This is also an issue that requires upstream changes. I though those changes
+were trivial, but apparently, they require bytecode changes. I tried to make a
+fix, that is half backed
+(https://github.com/ljmf00/llvm-project/tree/add-di-column-type) and currently
+freezed, due to lack of knowledge on bytecode read/write on the LLVM part. You
+can follow up the discussion of this issue in either LDC and LLVM bugzilla:
+- https://github.com/ldc-developers/ldc/issues/3865
+- https://bugs.llvm.org/show_bug.cgi?id=52470
+
+## Calling convention in DMD/LDC is wrong
+
+Calling convention in D compilers is currently behaving wrong, and we need to
+fix this.
+
+I decided to investigate how D calling convention works, in order to thinker
+with calling a function on debuggers. I thought D was using the standard
+calling convention with additional features like hidden parameters for context
+pointers, but I was wrong.
+
+I tested a few things on [godbolt](https://godbolt.org/z/sWz4x37bb), and
+realized that the parameters are passed to the CPU registers in the reverse
+order. I pushed a fix on the specification, thinking that we use a custom
+calling convention, since LDC is also relying on this behaviour, but apparently
+all the implementations are not conformant and the specification is right.
+
+### What does this means in terms of debugging?
+
+Well, with the wrong calling convention, debuggers can't reliably call
+functions and undefined behaviour happens, since the parameters are passed to
+the wrong registers. This is not a blocker for what I'm currently planning, but
+should definitely be fixed, in order to go forward with that feature, plus,
+binaries generated by different compilers can't be reliably linked.
+
+You can check the specification PR I made and appreciate discussion about that
+topic [here](https://github.com/dlang/dlang.org/pull/3120) .
+
+## Other reported issues and trivial patches
+
+I also reported the following issues:
+
+- https://issues.dlang.org/show_bug.cgi?id=22492
+- https://issues.dlang.org/show_bug.cgi?id=22493
+
+I've made some other trivial patches not worth much attention:
+- https://reviews.llvm.org/D113604
+- https://reviews.llvm.org/D113605
+- https://reviews.llvm.org/D113572
+- https://reviews.llvm.org/D113631
+
+## What is next?
+
+Regarding all these DWARF issues, I'm going to continue fixing some of them in
+parallel and probably going to start implementing the D TypeSystem and
+DWARFFASTParser, required for the LLDB plugin. These are things not trivial to
+me, but I'm going to try to have some output.
+
+Read about the [previous week](../d-saoc-2021-07/).
