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<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">Jeremy
wrote:</span></span></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important">>llvm-locstats
/ llvm-dwarfdump --statistics should probably be taught</span><br>
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important">>to
look at the inlined subroutines abstract origin to find all</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>variables,
however, it seems unwise to have internal compiler state</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>reflected
in the output file in the way it is above. The cause of the</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>empty
DW_TAG_formal_parameter being created in</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>DwarfDebug::collectEntityInfo
[0] -- it distinguishes between a</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>variable
that has no location intrinsics, and a variable that has only</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>empty
location intrinsics. Putting a filter in to skip variables with</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>only
empty locations avoids the difference in output, and reduce the</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>size
of .debug_info on a stage2reldeb clang build by about 20Mb, or</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>~1%.</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important">></span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>To
ensure this email contains a question: would there be any</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>objections
to adding that filter, and obliging consumers to look in</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>the
inlined subroutines abstract origin to find optimised-out</span><br style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255)">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><span style="background-color:rgb(255, 255, 255);display:inline !important">></span>variables,
instead of giving them a list per-inlined-instance?</span><br>
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<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><br>
</span></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important">+1
for resolving this at compiler level. I think <span style="background-color:rgb(255, 255, 255);display:inline !important">llvm-locstats / llvm-dwarfdump --statistics<span> displays good output for this case.</span></span></span></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important"><br>
</span></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important">Thanks,</span></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
<span style="color:rgb(32, 31, 30);font-family:"Segoe UI", "Segoe UI Web (West European)", "Segoe UI", -apple-system, BlinkMacSystemFont, Roboto, "Helvetica Neue", sans-serif;font-size:14.6667px;background-color:rgb(255, 255, 255);display:inline !important">Djordje</span></div>
<div style="font-family: Calibri, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);">
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Jeremy Morse via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> Wednesday, January 27, 2021 4:26 PM<br>
<b>To:</b> David Blaikie <dblaikie@gmail.com>; Paul Robinson <paul.robinson@sony.com>; Tozer, Stephen <Stephen.Tozer@sony.com>; llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> [llvm-dev] [DebugInfo] Different representations of optimised-out variables in DWARF</font>
<div> </div>
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<div class="BodyFragment"><font size="2"><span style="font-size:11pt;">
<div class="PlainText">Hi,<br>
<br>
This was "[llvm-dev] [DebugInfo] The current status of debug values<br>
using multiple machine locations" but I don't want to de-rail Stephens<br>
thread,<br>
<br>
Paul wrote:<br>
> I'm not actually sure what causes variables to be dropped from the DWARF<br>
> entirely, as opposed to them existing but having an unknown location for<br>
> their entire scope; however, outside of our desire to use dwarfdump to<br>
> analyze our debug info it's simply more efficient to omit variables with no<br>
> location, since they inflate the debug info size and I don't believe<br>
> there's any practical value in having them.<br>
<br>
David wrote:<br>
> When does this ^ happen? In optimized builds we include all local variables<br>
> in a "variables" attachment to the DISubprogram, so we shouldn't be losing<br>
> variables entirely.<br>
> [...]<br>
> I think it's pretty important that we keep them. It helps a user understand<br>
> that they've not mistyped the name of a variable, etc [...]<br>
<br>
This is something that's bothered me for a while, as it messes with<br>
our statistics when changing how variable locations are tracked. Take<br>
this completely contrived C file:<br>
<br>
int foo(int bar) {<br>
int baz = 12 + bar;<br>
return baz;<br>
}<br>
<br>
int qux(int quux) {<br>
int xyzzy = foo(quux);<br>
return xyzzy;<br>
}<br>
<br>
Using clang ef0dcb50630 and options "-O3 -g -c", llvm-locstats reports<br>
the object file has five variables in it. If you emit LLVM-IR, and<br>
replace the first operand of all "llvm.dbg.value" intrinsic<br>
invocations with "undef" and compile the IR with llc, then<br>
llvm-locstats still reports five variables. However: if you instead<br>
/delete/ all the invocations of "llvm.dbg.value", four variables are<br>
reported by llvm-locstats. This indicates there's an observable<br>
difference in the way we represent optimised-out variables in DWARF.<br>
<br>
The difference between the object files is the way they represent the<br>
inlined copy of "foo", here's the output with undef dbg.values,<br>
followed by the output when I delete all the dbg.value intrinsics:<br>
<br>
DW_TAG_inlined_subroutine<br>
DW_AT_abstract_origin (0x0000004e "foo")<br>
DW_AT_low_pc (0x0000000000000010)<br>
DW_AT_high_pc (0x0000000000000013)<br>
DW_AT_call_file ("/tmp/test.c")<br>
DW_AT_call_line (7)<br>
DW_AT_call_column (0x0f)<br>
<br>
DW_TAG_formal_parameter<br>
DW_AT_abstract_origin (0x0000005a "bar")<br>
<br>
NULL<br>
<br>
and:<br>
<br>
DW_TAG_inlined_subroutine<br>
DW_AT_abstract_origin (0x00000048 "foo")<br>
DW_AT_low_pc (0x0000000000000010)<br>
DW_AT_high_pc (0x0000000000000013)<br>
DW_AT_call_file ("/tmp/test.c")<br>
DW_AT_call_line (7)<br>
DW_AT_call_column (0x0f)<br>
<br>
NULL<br>
<br>
When there are dbg.value intrinsics present, then the inlined<br>
subroutine gets an empty DW_TAG_format_parameter that links back to<br>
the abstract origin. If there are no dbg.value intrinsics present, it<br>
does not. As far as I understand it, consumers can still determine<br>
that "bar" exists in the inlined subroutine by looking at the inlined<br>
subroutines abstract origin. This is what the "retained nodes"<br>
collection preserves.<br>
<br>
llvm-locstats / llvm-dwarfdump --statistics should probably be taught<br>
to look at the inlined subroutines abstract origin to find all<br>
variables, however, it seems unwise to have internal compiler state<br>
reflected in the output file in the way it is above. The cause of the<br>
empty DW_TAG_formal_parameter being created in<br>
DwarfDebug::collectEntityInfo [0] -- it distinguishes between a<br>
variable that has no location intrinsics, and a variable that has only<br>
empty location intrinsics. Putting a filter in to skip variables with<br>
only empty locations avoids the difference in output, and reduce the<br>
size of .debug_info on a stage2reldeb clang build by about 20Mb, or<br>
~1%.<br>
<br>
To ensure this email contains a question: would there be any<br>
objections to adding that filter, and obliging consumers to look in<br>
the inlined subroutines abstract origin to find optimised-out<br>
variables, instead of giving them a list per-inlined-instance?<br>
<br>
[0] <a href="https://github.com/llvm/llvm-project/blob/70e251497c4e26f8cfd85e745459afff97c909ce/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp#L1779">
https://github.com/llvm/llvm-project/blob/70e251497c4e26f8cfd85e745459afff97c909ce/llvm/lib/CodeGen/AsmPrinter/DwarfDebug.cpp#L1779</a><br>
<br>
--<br>
Thanks,<br>
Jeremy<br>
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