[lld-macho] Put GOT into `__DATA` segment where appropriate

[int3]

Summary

We were previously always emitting the GOT into __DATA_CONST, even for
target platforms where it should end up in __DATA.

I stumbled onto this while trying to use the class-dump tool -- with
the wrong segment names, it fails to locate the ObjC runtime info and
therefore fails to dump any classes.

[oontvoo]

lld/MachO/SyntheticSections.cpp

262

it's still not quite correct to unconditionally put it in non-const, though.
(there's the -data_const flag for one )

262

P.S: I dont actually have a strong opinion either way. But could we at least have a FIXME or note here for future work?

[int3]

lld/MachO/SyntheticSections.cpp

262

that's already being done in initializeSectionRenameMap in Driver.cpp :)

the problem was that we would only rename from non-const to const, and not the other way around. So initializing the section to a const segment here meant that we would never emit it as non-const...

[oontvoo]

LGTM

Looks like treat-undef-sym.s is failing . PTAL

lld/MachO/SyntheticSections.cpp

262

Ah, I see. Sorry for the noise then!

lld/MachO/SyntheticSections.cpp

 
 void NonLazyPointerSectionBase::writeTo(uint8_t *buf) const {
   for (size_t i = 0, n = entries.size(); i < n; ++i)
     if (auto *defined = dyn_cast<Defined>(entries[i]))
       write64le(&buf[i * target->wordSize], defined->getVA());
 }
 
 GotSection::GotSection()
-    : NonLazyPointerSectionBase(segment_names::dataConst, section_names::got) {
+    : NonLazyPointerSectionBase(segment_names::data, section_names::got) {
   flags = S_NON_LAZY_SYMBOL_POINTERS;
 }
 
 TlvPointerSection::TlvPointerSection()
     : NonLazyPointerSectionBase(segment_names::data,
                                 section_names::threadPtrs) {
   flags = S_THREAD_LOCAL_VARIABLE_POINTERS;
 }

lld/test/MachO/builtin-rename.s

 # YDATA-DAG: __DATA_CONST,__objc_classlist __DATA__objc_classlist
 # YDATA-DAG: __DATA_CONST,__objc_nlclslist __DATA__objc_nlclslist
 # YDATA-DAG: __DATA_CONST,__objc_catlist __DATA__objc_catlist
 # YDATA-DAG: __DATA_CONST,__objc_nlcatlist __DATA__objc_nlcatlist
 # YDATA-DAG: __DATA_CONST,__objc_protolist __DATA__objc_protolist
 # YDATA-DAG: __DATA_CONST,__objc_imageinfo __DATA__objc_imageinfo
 # YDATA-DAG: __DATA_CONST,__nl_symbol_ptr __IMPORT__pointers
 
+## LLD doesn't support defining symbols in synthetic sections, so we test them
+## via this slightly more awkward route.
+# RUN: llvm-readobj --section-headers %t/ydata | \
+# RUN:     FileCheck %s --check-prefix=SYNTH -DSEGNAME=__DATA_CONST
+# RUN: llvm-readobj --section-headers %t/ndata | \
+# RUN:     FileCheck %s --check-prefix=SYNTH -DSEGNAME=__DATA
+# RUN: llvm-readobj --section-headers %t/nopie | \
+# RUN:     FileCheck %s --check-prefix=SYNTH -DSEGNAME=__DATA
+# RUN: llvm-readobj --section-headers %t/old | \
+# RUN:     FileCheck %s --check-prefix=SYNTH -DSEGNAME=__DATA
+
+# SYNTH:      Name: __got
+# SYNTH-NEXT: Segment: [[SEGNAME]] ({{.*}})
+## Note that __la_symbol_ptr always remains in the non-const data segment.
+# SYNTH:      Name: __la_symbol_ptr
+# SYNTH-NEXT: Segment: __DATA ({{.*}})
+
 #--- renames.s
 .section __DATA,__auth_got
 .global __DATA__auth_got
 __DATA__auth_got:
   .space 8
 
 .section __DATA,__auth_ptr
 .global __DATA__auth_ptr

 __DATA__const:
   .space 8
 
 .section __DATA,__cfstring
 .global __DATA__cfstring
 __DATA__cfstring:
   .space 8
 
-# FIXME: error: conflicts with synthetic section ...
-# FIXME: we can't explicitly define syms in synthetic sections
-# COM: .section __DATA,__got
-# COM: .global __DATA__got
-# COM: __DATA__got:
-# COM:   .space 8
-
 .section __DATA,__mod_init_func,mod_init_funcs
 .global __DATA__mod_init_func
 __DATA__mod_init_func:
   .space 8
 
 .section __DATA,__mod_term_func,mod_term_funcs
 .global __DATA__mod_term_func
 __DATA__mod_term_func:

 __DATA__objc_protolist:
   .space 8
 
 .section __DATA,__objc_imageinfo
 .global __DATA__objc_imageinfo
 __DATA__objc_imageinfo:
   .space 8
 
-# FIXME: error: conflicts with synthetic section ...
-# FIXME: we can't explicitly define syms in synthetic sections
-# COM: .section __DATA,__la_symbol_ptr,lazy_symbol_pointers
-# COM: .global __DATA__la_symbol_ptr
-# COM: __DATA__la_symbol_ptr:
-# COM:   .space 8
-
 .section __IMPORT,__pointers,non_lazy_symbol_pointers
 .global __IMPORT__pointers
 __IMPORT__pointers:
   .space 8
 
 .section __TEXT,__StaticInit
 .section __TEXT,__StaticInit
 .global __TEXT__StaticInit
 __TEXT__StaticInit:
   .space 8
 
 #--- main.s
 .text
 .global _main
 _main:
+  mov ___nan@GOTPCREL(%rip), %rax ## ensure the __got section is created
+  callq ___isnan ## ensure the __la_symbol_ptr section is created
   ret