[COFF] In MinGW mode, ignore relocations against a discarded section

When GCC produces a jump table as part of a comdat function, the
jump table itself is produced as plain non-comdat rdata section. When
linked with ld.bfd, all of those rdata sections are kept, with
relocations unchanged in the sections that refer to discarded comdat
sections.

This has been observed with at least GCC 5.x and 7.x.

Differential Revision: https://reviews.llvm.org/D52600

llvm-svn: 343422

lld/COFF/Chunks.cpp

@@ -351,7 +351,12 @@ void SectionChunk::writeTo(uint8_t *Buf) const {
           check(File->getCOFFObj()->getSymbol(Rel.SymbolTableIndex));
       StringRef Name;
       File->getCOFFObj()->getSymbolName(Sym, Name);
-      error("relocation against symbol in discarded section: " + Name);
+
+      // MinGW mode object files (built by GCC) can have leftover sections
+      // with relocations against discarded comdat sections. Such sections
+      // are left as is, with relocations untouched.
+      if (!Config->MinGW)
+        error("relocation against symbol in discarded section: " + Name);
       continue;
     }
     // Get the output section of the symbol for this relocation.  The output

lld/test/COFF/Inputs/comdat-jumptable2.s

@@ -0,0 +1,35 @@
+        .section .text@comdatfunc, "x"
+        .linkonce discard
+        .globl comdatfunc
+comdatfunc:
+        leaq .Ljumptable(%rip), %rax
+        movslq (%rax, %rcx, 4), %rcx
+        addq %rcx, %rax
+        jmp *%rax
+
+        .section .rdata, "dr"
+        .long 0xcccccccc
+.Ljumptable:
+        .long .Ltail1-.Ljumptable
+        .long .Ltail2-.Ljumptable
+        .long .Ltail3-.Ljumptable
+        .long 0xdddddddd
+
+        .section .text@comdatfunc, "x"
+# If assembled with binutils, the following line can be kept in:
+#       .linkonce discard
+.Ltail1:
+        movl $1, %eax
+        ret
+.Ltail2:
+        movl $2, %eax
+        ret
+.Ltail3:
+        movl $3, %eax
+        ret
+
+        .text
+        .globl otherfunc
+otherfunc:
+        call comdatfunc
+        ret

lld/test/COFF/comdat-jumptable.s

@@ -0,0 +1,70 @@
+# REQUIRES: x86
+# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t1.obj
+# RUN: llvm-mc -triple=x86_64-windows-gnu %S/Inputs/comdat-jumptable2.s -filetype=obj -o %t2.obj
+
+# RUN: llvm-objdump -s %t1.obj | FileCheck --check-prefix=OBJ1 %s
+# RUN: llvm-objdump -s %t2.obj | FileCheck --check-prefix=OBJ2 %s
+
+# RUN: lld-link -lldmingw -entry:main %t1.obj %t2.obj -out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=EXE %s
+
+# Test linking cases where comdat functions have an associated jump table
+# in a non-comdat rdata (which GCC produces for functions with jump tables).
+# In these cases, ld.bfd keeps all rdata sections, but the relocations that
+# refer to discarded comdat sections just are emitted as they were originally.
+
+# In real scenarios, the jump table .rdata section should be identical across
+# all object files; here it is different to illustrate more clearly what
+# the linker actually does.
+
+# OBJ1: Contents of section .rdata:
+# OBJ1:  0000 aaaaaaaa 14000000 1e000000 28000000
+# OBJ1:  0010 bbbbbbbb
+
+# OBJ2: Contents of section .rdata:
+# OBJ2:  0000 cccccccc 14000000 1e000000 28000000
+# OBJ2:  0010 dddddddd
+
+# EXE: Contents of section .rdata:
+# EXE:  140002000 aaaaaaaa 0c100000 12100000 18100000
+# EXE:  140002010 bbbbbbbb cccccccc 14000000 1e000000
+# EXE:  140002020 28000000 dddddddd
+
+
+        .section .text@comdatfunc, "x"
+        .linkonce discard
+        .globl comdatfunc
+comdatfunc:
+        leaq .Ljumptable(%rip), %rax
+        movslq (%rax, %rcx, 4), %rcx
+        addq %rcx, %rax
+        jmp *%rax
+
+        .section .rdata, "dr"
+        .long 0xaaaaaaaa
+.Ljumptable:
+        .long .Ltail1-.Ljumptable
+        .long .Ltail2-.Ljumptable
+        .long .Ltail3-.Ljumptable
+        .long 0xbbbbbbbb
+
+        .section .text@comdatfunc, "x"
+# If assembled with binutils, the following line can be kept in:
+#       .linkonce discard
+.Ltail1:
+        movl $1, %eax
+        ret
+.Ltail2:
+        movl $2, %eax
+        ret
+.Ltail3:
+        movl $3, %eax
+        ret
+
+
+        .text
+        .globl main
+main:
+        call comdatfunc
+        call otherfunc
+        ret