RFC pragma(breakpoint) by benjones · Pull Request #21758 · dlang/dmd

benjones · 2025-08-22T08:46:26Z

Here's a first pass at pragma(breakpoint) as I described in my dconf talk. So far, it's basically pragma(backtrace) with no interactive features. Curious what people think about this (is it useful? is it the right information?)

Here's the output from the new test:

Pragma Breakpoint: static else, before expanding the dependent template
Starting backtrace
compiler/test/compilable/pragmaBreakpoint.d(8): scope: T
compiler/test/compilable/pragmaBreakpoint.d(2): scope: T!3LU
compiler/test/compilable/pragmaBreakpoint.d(17): scope: __anonymous
compiler/test/compilable/pragmaBreakpoint.d: scope: pragmaBreakpoint
compiler/test/compilable/pragmaBreakpoint.d: scope: __anonymous
Pragma Breakpoint: static else, before expanding the dependent template
Starting backtrace
compiler/test/compilable/pragmaBreakpoint.d(8): scope: T
compiler/test/compilable/pragmaBreakpoint.d(2): scope: T!2LU
compiler/test/compilable/pragmaBreakpoint.d(9): scope: __anonymous
compiler/test/compilable/pragmaBreakpoint.d: scope: pragmaBreakpoint
compiler/test/compilable/pragmaBreakpoint.d: scope: __anonymous
Pragma Breakpoint: static else, before expanding the dependent template
Starting backtrace
compiler/test/compilable/pragmaBreakpoint.d(8): scope: T
compiler/test/compilable/pragmaBreakpoint.d(2): scope: T!1LU
compiler/test/compilable/pragmaBreakpoint.d(9): scope: __anonymous
compiler/test/compilable/pragmaBreakpoint.d: scope: pragmaBreakpoint
compiler/test/compilable/pragmaBreakpoint.d: scope: __anonymous
Pragma Breakpoint: done T!0LU
Starting backtrace
compiler/test/compilable/pragmaBreakpoint.d(6): scope: T
compiler/test/compilable/pragmaBreakpoint.d(2): scope: T!0LU
compiler/test/compilable/pragmaBreakpoint.d(9): scope: __anonymous
compiler/test/compilable/pragmaBreakpoint.d: scope: pragmaBreakpoint
compiler/test/compilable/pragmaBreakpoint.d: scope: __anonymous
Pragma Breakpoint: main
Starting backtrace
compiler/test/compilable/pragmaBreakpoint.d(21): scope: __anonymous
compiler/test/compilable/pragmaBreakpoint.d(15): scope: __anonymous
compiler/test/compilable/pragmaBreakpoint.d(15): scope: pragmaBreakpoint
compiler/test/compilable/pragmaBreakpoint.d: scope: __anonymous

dlang-bot · 2025-08-22T08:46:30Z

Thanks for your pull request and interest in making D better, @benjones! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

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New or modified functions have Ddoc comments (with Params: and Returns:)

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Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#21758"

benjones · 2025-08-22T09:15:16Z

more (potential) discussion here: https://forum.dlang.org/post/bfznqduaojatgfxunygl@forum.dlang.org

benjones · 2025-10-07T17:47:43Z

I've added the outline of making this interactive:

./generated/osx/release/64/dmd compiler/test/runnable/pragmaBreakpoint.d
Pragma Breakpoint: static else, before expanding the dependent template
(ctdb) bt
Backtrace
compiler/test/runnable/pragmaBreakpoint.d(45): scope: T
compiler/test/runnable/pragmaBreakpoint.d(39): scope: T!3LU
compiler/test/runnable/pragmaBreakpoint.d(54): scope: __anonymous
compiler/test/runnable/pragmaBreakpoint.d: scope: pragmaBreakpoint
compiler/test/runnable/pragmaBreakpoint.d: scope: __anonymous
(ctdb) c
continuing compilation
Pragma Breakpoint: static else, before expanding the dependent template
(ctdb) c
continuing compilation
Pragma Breakpoint: static else, before expanding the dependent template
(ctdb) c
continuing compilation
Pragma Breakpoint: done T!0LU
(ctdb) c
continuing compilation
Pragma Breakpoint: main
(ctdb) c
continuing compilation

I assume there's a way to add this to the test above to make it run non-interactively?

/Users/ben/projects/dlang/dmd/dmd $ cat > pragmaMessageInputs.txt <<- EOM
heredocd> bt
heredocd> c
heredocd> c
heredocd> c
heredocd> c
heredocd> c
heredocd> EOM
/Users/ben/projects/dlang/dmd/dmd $ ./generated/osx/release/64/dmd compiler/test/runnable/pragmaBreakpoint.d < pragmaMessageInputs.txt
Pragma Breakpoint: static else, before expanding the dependent template
(ctdb) Backtrace
compiler/test/runnable/pragmaBreakpoint.d(45): scope: T
compiler/test/runnable/pragmaBreakpoint.d(39): scope: T!3LU
compiler/test/runnable/pragmaBreakpoint.d(54): scope: __anonymous
compiler/test/runnable/pragmaBreakpoint.d: scope: pragmaBreakpoint
compiler/test/runnable/pragmaBreakpoint.d: scope: __anonymous
(ctdb) continuing compilation
Pragma Breakpoint: static else, before expanding the dependent template
(ctdb) continuing compilation
Pragma Breakpoint: static else, before expanding the dependent template
(ctdb) continuing compilation
Pragma Breakpoint: done T!0LU
(ctdb) continuing compilation
Pragma Breakpoint: main
(ctdb) continuing compilation

Thoughts/feedback? Next thing to add will be something to print symbols in the various scopes of the backtrace.

benjones · 2026-10-23T21:41:46Z

Finally had some time to come back to this. Any feedback on the functionality? There's definitely some improvements to be made to the implementation, but I'll hold off on them until I'm confident that this is headed in the right direction. I can't tell if my test is causing the CI to hang, or if it's normal issues (looks to be hanging on ... fail_compilation/cast_qual.d -verrors=simple -verrors=0 -preview=dip1000 -de -fPIC () on the Ubuntu runners which makes me think it's not an issue with my new dshell test, but I can't use the runner locally since I'm on an ARM mac...)

github-actions · 2026-10-27T17:48:11Z

DMD perf check

Metric	Base	PR	Δ
compile dmd itself (wall)	10.6 s	9.7 s	-8.49%

All measurements

Metric	Base	PR	Δ
compile hello.d (instr)	203.5 M	203.5 M	+0.008%
compile hello.d -O -release (instr)	221.1 M	221.1 M	+0.015%
compile Phobos (instr)	4,709.6 M	4,710.1 M	+0.010%
compile Phobos codegen (instr)	1,421.6 M	1,421.8 M	+0.013%
compile vibe.d (instr)	13,564.2 M	13,569.9 M	+0.042%
compile ctRegex tests (instr)	29,185.5 M	29,188.9 M	+0.012%
dmd binary size (stripped)	8.18 MB	8.18 MB	0.00%
hello binary size (stripped)	0.72 MB	0.72 MB	0.00%
peak RSS (compile hello.d)	44.39 MB	44.30 MB	-0.19%
peak RSS (compile Phobos)	610.6 MB	611.3 MB	+0.11%
peak RSS (compile vibe.d)	1879 MB	1879 MB	-0.02%
peak RSS (compile ctRegex tests)	2040 MB	2042 MB	+0.07%
page faults (compile hello.d)	8,732	8,729	-0.03%
page faults (compile Phobos)	152,347	151,986	-0.24%
page faults (compile vibe.d)	470,098	470,034	-0.01%
page faults (compile ctRegex tests)	497,815	498,202	+0.08%
compile dmd itself (wall)	10.6 s	9.7 s	-8.49%
compile ctRegex tests (wall)	3,263 ms	3,181 ms	-2.53%
compile hello.d (wall)	52.8 ms	71.8 ms	+35.91%
compile Phobos (wall)	1,188 ms	1,181 ms	-0.51%

_{798ca1e vs merge-base 21cf5b1 · about these metrics · historical dashboard}

thewilsonator · 2026-10-27T23:20:15Z

+        .map!(l => l.replace("\\", "/")) //paths are different on different platforms.  TODO, use module name instead?
+        .filter!(l => l.length > 0)
+        .join("\n");
+    assert(output == expectedOutput,  "actual output:\n" ~ output ~ "\nexpected output:\n" ~ expectedOutput);


ERROR: Found the following residual temporary files. ERROR: Temporary files should be stored in `test_results` or explicitly removed. ?? compiler/test/pragmabreakpoint

thewilsonator · 2026-10-27T23:20:57Z

+    while(true)
+    {
+        auto result = tryWait(proc);
+        if(result.terminated){


spaces between if and opening (

thewilsonator · 2026-10-27T23:21:05Z

+            enforce(result.status == 0);
+            break;
+        }
+        if(MonoTime.currTime > deadline){


thewilsonator · 2026-10-27T23:21:18Z

+    inputPipe.writeEnd.write("bt\nm 3\nc\nc\nc\nbt\nm 1\nc\nc\nc\nc\n"); //2 extra "continue" + newlines should be ignored
+    inputPipe.writeEnd.flush();
+    const deadline = MonoTime.currTime + 5.seconds;
+    while(true)


thewilsonator · 2026-10-28T01:23:51Z

This looks good to me, you might want to bring this up at the Foundation monthly.

benjones · 2026-10-28T15:25:12Z

This looks good to me, you might want to bring this up at the Foundation monthly.

I'll fix the issues you brought up later today, hopefully. When is the next meeting? I've never joined one before (my email is ben.james.jones@gmail.com if you want to take this out of the PR comments)

benjones marked this pull request as draft August 22, 2025 08:46

thewilsonator added the Severity:Enhancement label Aug 22, 2025

benjones force-pushed the pragmaBreakpoint branch from b16de19 to 7dda94d Compare August 22, 2025 09:14

thewilsonator added Review:Needs Changelog A changelog entry needs to be added to /changelog Review:Needs Spec Change A change updating the language specification needs to be submitted as part of this PR labels Aug 22, 2025

benjones force-pushed the pragmaBreakpoint branch 2 times, most recently from 7c31bc2 to 26e1f78 Compare August 22, 2025 10:17

benjones force-pushed the pragmaBreakpoint branch from 26e1f78 to ee7e2e9 Compare October 7, 2025 17:45

benjones force-pushed the pragmaBreakpoint branch from ee7e2e9 to 2d455ef Compare October 7, 2025 17:48

benjones added 5 commits October 23, 2026 11:29

first pass at pragma breakpoint

10c665e

make pragma breakpoint interactive

3796504

turn test into compilable shell test to run it without interactivity

a494423

start backtrace impl

85f12c9

update test to use dshell

e592769

benjones force-pushed the pragmaBreakpoint branch from 9acb6b3 to e592769 Compare October 23, 2026 17:35

fix 32 bit compiler error

79fda6e

benjones force-pushed the pragmaBreakpoint branch 2 times, most recently from 223404d to 668aec8 Compare October 23, 2026 21:08

benjones force-pushed the pragmaBreakpoint branch 8 times, most recently from 12b890e to 3019896 Compare October 24, 2026 20:03

benjones force-pushed the pragmaBreakpoint branch from 3019896 to cb7a451 Compare October 24, 2026 23:56

fix test

2949e78

benjones force-pushed the pragmaBreakpoint branch from cb7a451 to 2949e78 Compare October 25, 2026 00:29

thewilsonator reviewed Oct 25, 2026

View reviewed changes

Comment thread compiler/src/dmd/common/stringutil.d Outdated

benjones force-pushed the pragmaBreakpoint branch 8 times, most recently from bb15867 to 64dc4df Compare October 27, 2026 16:43

move split to dmd.root.string

48c7987

benjones force-pushed the pragmaBreakpoint branch from 64dc4df to 48c7987 Compare October 27, 2026 16:56

add changelog

798ca1e

benjones force-pushed the pragmaBreakpoint branch from c4e255b to 798ca1e Compare October 27, 2026 17:33

thewilsonator reviewed Oct 27, 2026

View reviewed changes

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RFC pragma(breakpoint) - #21758

RFC pragma(breakpoint)#21758
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