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<div class="moz-cite-prefix">24.09.2019 8:26, Rui Ueyama пишет:<br>
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<blockquote type="cite"
cite="mid:CAJENXgtaOHj1Ff=E_NZHm1n6dz3sGBiv48ram5yWxgPrK2O-bw@mail.gmail.com">
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<div>Hi Alexey,</div>
<div><br>
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<div>Thank you for sharing this proposal. Reducing the size of
debug info is generally a good thing, and I believe you'd see
more debug info size reduction in Rust programs than in C++
programs, because I heard that the Rust compiler driver passes
a lot of object files to the linker, expecting that the linker
would remove most of them, which leaves dead debug info.</div>
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<p>Hi Rui, Thanks!<br>
</p>
<blockquote type="cite"
cite="mid:CAJENXgtaOHj1Ff=E_NZHm1n6dz3sGBiv48ram5yWxgPrK2O-bw@mail.gmail.com">
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<div dir="ltr">On Thu, Sep 12, 2019 at 7:32 AM Alexey Lapshin
via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org"
moz-do-not-send="true">llvm-dev@lists.llvm.org</a>>
wrote:<br>
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0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div>Debuginfo and linker folks, we (AccessSoftek) would
like to suggest a proposal for removing obsolete debug
info. If you find it useful we will be happy to work on
improving it. Thank you for any opinions and suggestions.<br>
<br>
Alexey.<br>
<br>
Currently when the linker does garbage collection a
lot of abandoned debug info is left behind (see Appendix A
for documentation). Besides inflated debug info size, we
ended up with overlapping address ranges and no way to say
valid vs garbage ranges. We propose removing debug info
along with removing code. This would reduce debug info
size and make sure debug info accuracy.<br>
<br>
There are several approaches which could be used to solve
that problem:<br>
<br>
1. Require dwarf producers to generate fragmented debug
data according to DWARF5 specification: "E.3.3
Single-function-per-DWARF-compilation-unit" page 388. That
approach assumes fragmenting the whole debug info per
function basis and glue fragmented sections at the link
time using section groups.<br>
<br>
2. Use an additional tool, which would optimize out
unnecessary debug data, something similar to dwz (dwarf
compressor tool), dsymutil (links the DWARF debug
information). This approach assumes additional post-link
binaries processing.<br>
<br>
3. Teach the linker to parse debug data and let it remove
unused debug data. <br>
<br>
In this proposal, we focus on approach #3. We show that
this approach is viable and discuss some preliminary
results, leaving particular implementation out of the
scope. We attach the Proof of Concept (PoC)
implementation(<a href="https://reviews.llvm.org/D67469"
target="_blank" moz-do-not-send="true">https://reviews.llvm.org/D67469</a>)
for illustrative purposes. Please keep in mind that it is
not final, and there is room for improvements (see
Appendix B). However, the achieved results look quite
promising and demonstrate up to 2 times size reduction and
performance overhead is 30% of linking time (which is in
the same ballpark as the already done section compressing
(see table 2 point F)).<br>
</div>
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<div><br>
</div>
<div>I believe #1 was added to DWARF5 to make link-time debug
info GC possible, so could you tell me a little bit about
why you chose to do #3? Is this because you want to do this
for DWARF4?</div>
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<p><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">No, that
proposal is not DWARF-4 specific. The proposal is for DWARF-5
also. The solution added to DWARF-5("E.3.3
Single-function-per-DWARF-compilation-unit" page 388.) is not a
complete solution. This is a recommendation which needs to have
an additional specification.</span><br style="color: rgb(0, 0,
0); font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 16px; font-style: normal; font-variant-ligatures:
normal; font-variant-caps: normal; font-weight: 400;
letter-spacing: normal; orphans: 2; text-align: start;
text-indent: 0px; text-transform: none; white-space: normal;
widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px;
background-color: rgb(255, 255, 255); text-decoration-style:
initial; text-decoration-color: initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">There is
-fdebug-types-section implementation which follows that
recommendation. Other cases(other than type units) do not
easily fit into this recommendation. There are tables which have
a common header. F.e. .debug_line, .debug_rnglists, .debug_addr.
It is not clear how these tables could be separated between
section groups.</span><br style="color: rgb(0, 0, 0);
font-family: Calibri, Arial, Helvetica, sans-serif; font-size:
16px; font-style: normal; font-variant-ligatures: normal;
font-variant-caps: normal; font-weight: 400; letter-spacing:
normal; orphans: 2; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: 2;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
background-color: rgb(255, 255, 255); text-decoration-style:
initial; text-decoration-color: initial;">
<br style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">The more
important thing is the fragmentation itself. Dividing debug
tables into pieces would increase debug info size.</span><br
style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">It also would
significantly complicate code working with debug info. F.e.
include/llvm/DebugInfo/DWARF/DWARFObject.h has interface for
class DWARFObject. </span><span style="color: rgb(0, 0, 0);
font-family: Calibri, Arial, Helvetica, sans-serif; font-size:
16px; font-style: normal; font-variant-ligatures: normal;
font-variant-caps: normal; font-weight: 400; letter-spacing:
normal; orphans: 2; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: 2;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
background-color: rgb(255, 255, 255); text-decoration-style:
initial; text-decoration-color: initial; display: inline
!important; float: none;">It currently is not ready for the case
when there could be multiple tables. Patch introducing support
for multiple tables would be massive change affected many places
in llvm codebase. <br>
</span></p>
<p><span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">Another thing
is that not only the llvm code base but all other DWARF
consumers should be changed to support fragmented debug info. </span><br
style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<br style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">Shortly, if
all debug tables would be fragmented then working with debug
info would be significantly complicated.</span><br style="color:
rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica,
sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<br style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">Thus the
reasons to select #3 are :</span><br style="color: rgb(0, 0, 0);
font-family: Calibri, Arial, Helvetica, sans-serif; font-size:
16px; font-style: normal; font-variant-ligatures: normal;
font-variant-caps: normal; font-weight: 400; letter-spacing:
normal; orphans: 2; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: 2;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
background-color: rgb(255, 255, 255); text-decoration-style:
initial; text-decoration-color: initial;">
<br style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">1. It could
be done in a single place, not affecting other parts of the llvm
code base.</span><br style="color: rgb(0, 0, 0); font-family:
Calibri, Arial, Helvetica, sans-serif; font-size: 16px;
font-style: normal; font-variant-ligatures: normal;
font-variant-caps: normal; font-weight: 400; letter-spacing:
normal; orphans: 2; text-align: start; text-indent: 0px;
text-transform: none; white-space: normal; widows: 2;
word-spacing: 0px; -webkit-text-stroke-width: 0px;
background-color: rgb(255, 255, 255); text-decoration-style:
initial; text-decoration-color: initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">2. It does
not require other DWARF consumers to implement support for it.</span><br
style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">3. Avoiding
fragmentation would save space.</span><br style="color: rgb(0,
0, 0); font-family: Calibri, Arial, Helvetica, sans-serif;
font-size: 16px; font-style: normal; font-variant-ligatures:
normal; font-variant-caps: normal; font-weight: 400;
letter-spacing: normal; orphans: 2; text-align: start;
text-indent: 0px; text-transform: none; white-space: normal;
widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px;
background-color: rgb(255, 255, 255); text-decoration-style:
initial; text-decoration-color: initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">4. Processing
of not fragmented debug info is faster.</span><br style="color:
rgb(0, 0, 0); font-family: Calibri, Arial, Helvetica,
sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;">5. No need to
adapt DWARF tables for fragmentation. They could be handled with
their current state.</span><br>
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255, 255,
255); text-decoration-style: initial; text-decoration-color:
initial; display: inline !important; float: none;"><br
style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255,
255, 255); text-decoration-style: initial;
text-decoration-color: initial;">
<span style="color: rgb(0, 0, 0); font-family: Calibri, Arial,
Helvetica, sans-serif; font-size: 16px; font-style: normal;
font-variant-ligatures: normal; font-variant-caps: normal;
font-weight: 400; letter-spacing: normal; orphans: 2;
text-align: start; text-indent: 0px; text-transform: none;
white-space: normal; widows: 2; word-spacing: 0px;
-webkit-text-stroke-width: 0px; background-color: rgb(255,
255, 255); text-decoration-style: initial;
text-decoration-color: initial; display: inline !important;
float: none;"><br>
</span></span></p>
<p>Alexey<br>
</p>
<blockquote type="cite"
cite="mid:CAJENXgtaOHj1Ff=E_NZHm1n6dz3sGBiv48ram5yWxgPrK2O-bw@mail.gmail.com">
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0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div><br>
<br>
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