https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68771
Bug ID: 68771
Summary: Darwin: Profile guided optimisation with cold sections
and invalid symbol redefinition
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: debug
Assignee: unassigned at gcc dot gnu.org
Reporter: zerolo at gmail dot com
Target Milestone: ---
Created attachment 36944
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=36944&action=edit
Patch to make cold section label unique(r?)
I'm not sure which component to attach this report to, the problem seems to be
specific to Darwin (and stem from its format for debug symbols but is present
whether generating debug info or not), but relates to both PGO (which causes
the cold and hot splits) and is exacerbate by LTO.
On the Darwin platform, a label is inserted for debugging / EH purposes when a
function is split into hot / hold (with info generated from PGO).
Rough repo steps for Mac OS 10.11.1 El Yosemite with Homebrew gcc 5.3.0
1) Compile/link C++ project with -Ofast -DNDEBUG -flto -fprofile-generate
2) Run to generate profiling files
3) Compile/link C++ project with -Ofast -DNDEBUG -flto -fprofile-use
Step 3's link aborts on multiple temporary assembler files with errors such as
/var/folders/02/yl3m8d4d0397mk6dxn6dpcqw0000gp/T//ccgBAKOU.s:3430:1: error:
invalid symbol redefinition
___cold_sect_of_ComputeBoundingBox:
^
/var/folders/02/yl3m8d4d0397mk6dxn6dpcqw0000gp/T//ccHiavMh.s:1652:1: error:
invalid symbol redefinition
___cold_sect_of_GetInterpolationMatrix:
^
/var/folders/02/yl3m8d4d0397mk6dxn6dpcqw0000gp/T//ccjQlEWp.s:11019:1: error:
invalid symbol redefinition
___cold_sect_of_GetReferencePoints:
^
The methods mentioned exist multiple times, but either as different overloads
(different C++ arguments, so distinct mangled name), or in completely
difference classes (or static in distinct files).
The machinery that actually generates those __cold_sect_of_ seems to be set up
in gcc/config/darwin.c:3682.
Originally this seems to come from a patch for debugging information here
https://gcc.gnu.org/ml/gcc-patches/2010-12/msg01609.html
These label are derived *only* from the name of the function, so it fails when
multiple overloads are present, also seems to cause problems with LTO.
When I change darwin.c to use DECL_ASSEMBLER_NAME instead (and increase the
buffer size for really long mangled names), the link works, although I'm not
sure about the accuracy of debug information.
