https://llvm.org/bugs/show_bug.cgi?id=28554
Bug ID: 28554
Summary: LLD fails to devirtualize the code that Gold plugin
does
Product: lld
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: ELF
Assignee: unassignedb...@nondot.org
Reporter: kra...@google.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 16742
--> https://llvm.org/bugs/attachment.cgi?id=16742&action=edit
devirt.cc
Consider the attached program, devirt.cc. With Gold and LLVM Gold plugin we
get:
$ clang++ -O2 -fuse-ld=gold -o lala -fvisibility=hidden devirt.cc -std=c++11
-flto -fwhole-program-vtables -Wl,-plugin-opt,-pass-remarks=wholeprogramdevirt
-Wl,-plugin-opt,O1 -Wl,-plugin-opt,-function-sections
-Wl,-plugin-opt,save-temps
LLVM gold plugin: <unknown>:0:0: devirtualized call
LLVM gold plugin: <unknown>:0:0: devirtualized call
LLVM gold plugin: <unknown>:0:0: devirtualized call
$
In the bitcode (lala.opt.bc) we see the call was devirtualized:
%vfn = getelementptr inbounds i1 (%class.Base*, i32)*, i1 (%class.Base*,
i32)** %vtable, i64 2
%10 = load i1 (%class.Base*, i32)*, i1 (%class.Base*, i32)** %vfn, align 8
%11 = icmp ne i8* %9, getelementptr (i8, i8* bitcast ([6 x i8*]* @_ZTV1A to
i8*), i64 16)
%conv = zext i1 %11 to i32
%call19 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x
i8], [4 x i8]* @.str.1, i64 0, i64 0), i32 %conv)
If we link with lld, nothing happens:
$ clang++ -fuse-ld=lld -o lala devirt.cc -std=c++11 -flto -fvisibility=hidden
-fwhole-program-vtables -Wl,-mllvm,-pass-remarks=wholeprogramdevirt
-Wl,--lto-O1 -Wl,-save-temps
$
And the bitcode shows that we still have a virtual call:
%vfn = getelementptr inbounds i1 (%class.Base*, i32)*, i1 (%class.Base*,
i32)** %vtable, i64 2
%19 = load i1 (%class.Base*, i32)*, i1 (%class.Base*, i32)** %vfn, align 8
%call17 = invoke zeroext i1 %19(%class.Base* %call15, i32 1)
to label %invoke.cont16 unwind label %lpad
invoke.cont16: ; preds = %sw.epilog
%conv = zext i1 %call17 to i32
%call19 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([4 x i8],
[4 x i8]* @.str.1, i32 0, i32 0), i32 %conv)
This prevents lld from being deployed in Chromium as the default linker on
Linux.
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