[LLD] [COFF] Don't create pseudo relocations for discardable sections (#89043)

This extends on the case from 9c970d5;
if a section is marked discardable, it won't be mapped into memory at
runtime, so there's no point in creating runtime pseudo relocations for
such sections.

Author: mstorsjo

lld/COFF/Writer.cpp

@@ -2060,6 +2060,10 @@ void Writer::createRuntimePseudoRelocs() {
     auto *sc = dyn_cast<SectionChunk>(c);
     if (!sc || !sc->live)
       continue;
+    // Don't create pseudo relocations for sections that won't be
+    // mapped at runtime.
+    if (sc->header->Characteristics & IMAGE_SCN_MEM_DISCARDABLE)
+      continue;
     sc->getRuntimePseudoRelocs(rels);
   }
 

lld/test/COFF/autoimport-debug.s

@@ -0,0 +1,43 @@
+# REQUIRES: x86
+# RUN: split-file %s %t.dir
+
+## We've got references to variable both in a .refptr and in .debug_info.
+## The .debug_info section should be discardable, so no pseudo relocations
+## need to be created in it. The .refptr section should be elimiated
+## and redirected to __imp_variable instead, so we shouldn't need to
+## create any runtime pseudo relocations. Thus, test that we can link
+## successfully with -runtime-pseudo-reloc:no, while keeping the
+## debug info.
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.dir/lib.s -filetype=obj -o %t.dir/lib.obj
+# RUN: lld-link -out:%t.dir/lib.dll -dll -entry:DllMainCRTStartup %t.dir/lib.obj -lldmingw -implib:%t.dir/lib.lib
+
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.dir/main.s -filetype=obj -o %t.dir/main.obj
+# RUN: lld-link -lldmingw -out:%t.dir/main.exe -entry:main %t.dir/main.obj %t.dir/lib.lib -opt:noref -debug:dwarf -runtime-pseudo-reloc:no
+
+#--- main.s
+    .global main
+    .text
+main:
+    movq .refptr.variable(%rip), %rax
+    ret
+
+    .section .rdata$.refptr.variable,"dr",discard,.refptr.variable
+    .global .refptr.variable
+.refptr.variable:
+    .quad   variable
+
+    .section .debug_info
+    .long 1
+    .quad variable
+    .long 2
+
+#--- lib.s
+    .global variable
+    .global DllMainCRTStartup
+    .text
+DllMainCRTStartup:
+    ret
+    .data
+variable:
+    .long 42