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[NFC] Fix #106873 - update assignment tracking docs #106959

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Sep 2, 2024
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[NFC] Fix #106873 - update assignment tracking docs #106959

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[NFC] Fix #106873 - update assignment tracking docs
OCHyams Sep 2, 2024
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OCHyams Sep 2, 2024
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20 changes: 13 additions & 7 deletions llvm/docs/AssignmentTracking.md
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Expand Up @@ -20,12 +20,14 @@ LLVM pass writers, and minimal disruption to LLVM in general.

## Status and usage

**Status**: Experimental work in progress. Enabling is strongly advised against
except for development and testing.
**Status**: Enabled by default in Clang but disabled under some circumstances
(which can be overridden with the `forced` option, see below). `opt` will not
run the pass unless asked (`-passes=declare-to-assign`).

**Enable in Clang**: `-Xclang -fexperimental-assignment-tracking`
**Flag**:
`-Xclang -fexperimental-assignment-tracking=<disabled|enabled|forced>`

That causes Clang to get LLVM to run the pass `declare-to-assign`. The pass
When enabled Clang gets LLVM to run the pass `declare-to-assign`. The pass
converts conventional debug records to assignment tracking metadata and sets
the module flag `debug-info-assignment-tracking` to the value `i1 true`. To
check whether assignment tracking is enabled for a module call
Expand Down Expand Up @@ -88,7 +90,7 @@ int fun(int a) {
```
compiled without optimisations:
```
$ clang++ test.cpp -o test.ll -emit-llvm -S -g -O0 -Xclang -fexperimental-assignment-tracking
$ clang++ test.cpp -o test.ll -emit-llvm -S -g -O0 -Xclang -fexperimental-assignment-tracking=enabled
```
we get:
```
Expand Down Expand Up @@ -194,8 +196,7 @@ the choice at each instruction, iteratively joining the results for each block.

### TODO list

As this is an experimental work in progress so there are some items we still need
to tackle:
Outstanding improvements:

* As mentioned in test llvm/test/DebugInfo/assignment-tracking/X86/diamond-3.ll,
the analysis should treat escaping calls like untagged stores.
Expand Down Expand Up @@ -228,3 +229,8 @@ to tackle:
that we can only track assignments with fixed offsets and sizes, I think we
can probably get rid of the address and address-expression part, since it
will always be computable with the info we have.

* Assignment tracking is disabled by default for LTO and thinLTO builds, and
if LLDB debugger tuning has been specified. We should remove these
restrictions. See EmitAssemblyHelper::RunOptimizationPipeline in
clang/lib/CodeGen/BackendUtil.cpp.
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