https://llvm.org/bugs/show_bug.cgi?id=31052
Bug ID: 31052
Summary: ThinLTO + switch table + O2 cause undefined symbols
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: kra...@google.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 17607
--> https://llvm.org/bugs/attachment.cgi?id=17607&action=edit
reproducer
Consider the following program that consists of two files (the archive with the
sources is attached):
main.cc:
#include <stdio.h>
typedef int (*CallFunc) ();
CallFunc GetCallN(int i, int j);
int main(void) {
printf("%d\n", GetCallN(1, 1)());
printf("%d\n", GetCallN(2, 1)());
}
call.cc:
typedef int (*CallFunc) ();
static int CallX() { return 0; }
static int CallY() { return 1; }
const CallFunc Funcs0[] = {CallX, CallY, CallY, CallY};
const CallFunc Funcs1[] = {CallY, CallX, CallY, CallY};
const CallFunc Funcs2[] = {CallY, CallY, CallX, CallY};
CallFunc GetCallN(int i, int j) {
switch (i) {
case 0:
return Funcs0[j];
case 1:
return Funcs1[j];
case 2:
return Funcs2[j];
default:
break;
}
return 0;
}
Let's build it with ThinLTO and LLD:
clang++ -o main main.cc call.cc -O2 -fuse-ld=lld -flto=thin
The following error will be displayed:
/usr/local/google/home/krasin/src/llvm.org/release-build/bin/ld.lld: error:
/tmp/main-fa77ab.o0 (function main): undefined symbol 'CallX()
(.llvm.5BF168B0)'
/usr/local/google/home/krasin/src/llvm.org/release-build/bin/ld.lld: error:
/tmp/main-fa77ab.o0 (function main): undefined symbol 'CallY()
(.llvm.5BF168B0)'
clang-3.9: error: linker command failed with exit code 1 (use -v to see
invocation)
- It works correctly with FullLTO:
$ clang++ -o main main.cc call.cc -O2 -fuse-ld=lld -flto && ./main
0
1
- It works with a regular build (with lld, as well as ld).
- It works with FullLTO + Gold
- It fails with ThinLTO + Gold:
$ clang++ -o main main.cc call.cc -O2 -fuse-ld=gold -flto=thin
/tmp/lto-llvm-22f26f.o:/tmp/main-155d01.o:function main: error: undefined
reference to '_ZL5CallXv.llvm.5BF168B0'
/tmp/lto-llvm-22f26f.o:/tmp/main-155d01.o:function main: error: undefined
reference to '_ZL5CallYv.llvm.5BF168B0'
clang-3.9: error: linker command failed with exit code 1 (use -v to see
invocation)
Looking at the bitcode, -O2 causes the switch statement be converted into a
switch table:
@_ZL6Funcs0 = internal unnamed_addr constant [4 x i32 ()*] [i32 ()*
@_ZL5CallXv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallYv],
align 16
@_ZL6Funcs1 = internal unnamed_addr constant [4 x i32 ()*] [i32 ()*
@_ZL5CallYv, i32 ()* @_ZL5CallXv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallYv],
align 16
@_ZL6Funcs2 = internal unnamed_addr constant [4 x i32 ()*] [i32 ()*
@_ZL5CallYv, i32 ()* @_ZL5CallYv, i32 ()* @_ZL5CallXv, i32 ()* @_ZL5CallYv],
align 16
@switch.table = private unnamed_addr constant [3 x [4 x i32 ()*]*] [[4 x i32
()*]* @_ZL6Funcs0, [4 x i32 ()*]* @_ZL6Funcs1, [4 x i32 ()*]* @_ZL6Funcs2]
; Function Attrs: norecurse nounwind readonly uwtable
define i32 ()* @_Z8GetCallNii(i32 %i, i32 %j) local_unnamed_addr #0 {
entry:
%0 = icmp ult i32 %i, 3
br i1 %0, label %switch.lookup, label %return
switch.lookup: ; preds = %entry
%1 = sext i32 %i to i64
%switch.gep = getelementptr inbounds [3 x [4 x i32 ()*]*], [3 x [4 x i32
()*]*]* @switch.table, i64 0, i64 %1
%switch.load = load [4 x i32 ()*]*, [4 x i32 ()*]** %switch.gep, align 8
%idxprom5 = sext i32 %j to i64
%arrayidx6 = getelementptr inbounds [4 x i32 ()*], [4 x i32 ()*]*
%switch.load, i64 0, i64 %idxprom5
%2 = load i32 ()*, i32 ()** %arrayidx6, align 8, !tbaa !1
br label %return
return: ; preds = %switch.lookup,
%entry
%retval.0 = phi i32 ()* [ null, %entry ], [ %2, %switch.lookup ]
ret i32 ()* %retval.0
}
Whenever I change the code so that the switch table is not generated, it would
compile with ThinLTO just fine.
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