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microsoft / MIEngine

The Visual Studio MI Debug Engine ("MIEngine") provides an open-source Visual Studio Debugger extension that works with MI-enabled debuggers such as gdb and lldb.
MIT License
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[#1349] Always use `--simple-values` in newer versions of GDB. #1400

Closed gareth-rees closed 1 year ago

gareth-rees commented 1 year ago

Summary

In newer versions of GDB, the --simple-values option to the -stack-list-arguments, -stack-list-locals and -stack-list-variables commands no longer prints the value for references to compound types. This improves the performance of these commands when the stack has references to large data structures.

When these versions of GDB are available, take advantage by using --simple-values in DebuggedProcess.GetParameterInfoOnly to fetch names and types in a single -stack-list-arguments command. This is faster than the old approach of using --no-values followed with -var-create and -var-delete for each parameter to get the type.

The new method MICommandFactory.SupportsSimpleValuesExcludesRefTypes determines if the debugger supports the improved behaviour of the --simple-values option, by executing the -list-features command and checking for the "simple-values-ref-types" feature in the output. We cache the result on the DebuggedProcess object as the set of supported features does not change during the lifetime of the debug session.

Fixes #1349 (Unnecessary -var-create and -var-delete commands during a stack trace can cause noticeable pause each time the debuggee stops)

Details

The behaviour of GDB was changed in commit 51f8dafba8 and this change will be included in the next major release of GDB (version 14). I understand that Microsoft developers are not allowed to look at GDB's source code (see comment from Gregg Miskelly here), so I have summarized the effect of the change above. Let me know if there is anything else you'd like to know about this change.

I repeated the reproduction procedure from #1349, took a copy of the commands issued by a single Step Over operation with ten frames on the stack and attached them here: stack-list-arguments-after.gz. The table below shows the counts of GDB/MI commands issued for the Step Over operation by the MIEngine from main against GDB 13.1 and the MIEngine from this branch against the updated GDB.

Command Counts (main) Counts (this branch)
-stack-list-arguments 2 2
-stack-list-frames 1 1
-stack-list-variables 1 1
-stack-select-frame 20 0
-var-create 210 10
-var-delete 210 10
gareth-rees commented 1 year ago

@gregg-miskelly You looked at my previous attempt to fix this issue, pull request #1350, so it would valuable if you could take a look at this attempt too.

gregg-miskelly commented 1 year ago

@WardenGnaw do you want to take a look as well? These look good to me.

@gareth-rees FYI WardenGnaw is on vacation, and Monday is a holiday in the US, so we probably will not get back to you till Tuesday or Wednesday.

WardenGnaw commented 1 year ago

LGTM. Linux tests are just being flaky and not failing due to the changes.

Thanks again for contributing @gareth-rees!