[lld] r366780 - [COFF] Unbreak sorting of mingw comdat .tls sections after SVN r363457

[Martin Storsjo via llvm-commits]

Author: mstorsjo
Date: Mon Jul 22 23:38:04 2019
New Revision: 366780

URL: http://llvm.org/viewvc/llvm-project?rev=366780&view=rev
Log:
[COFF] Unbreak sorting of mingw comdat .tls sections after SVN r363457

Code built for mingw with -fdata-sections will store each TLS variable
in a comdat section, named .tls$$<varname>. Normal TLS variables are
stored in sections named .tls$ with a trailing dollar, which are
sorted after a starter marker (in a later linked object file) in a
section named ".tls" (with no dollar suffix), before an ending marker
in a section named ".tls$ZZZ".

The mingw comdat section suffix stripping introduced in SVN r363457
broke sorting of such tls sections, ending up sorting the stripped
.tls$$<varname> sections (stripped to ".tls") before the start marker
in the section named ".tls".

We could add exceptions to the section name suffix stripping for
.tls (and .CRT, where suffixes always should be honored), but the
more conservative option is probably the reverse; to only apply the
stripping for the normal sections where sorting shouldn't have any
effect.

Differential Revision: https://reviews.llvm.org/D65018

Added:
    lld/trunk/test/COFF/tls_suffix_sorting.s
Modified:
    lld/trunk/COFF/Writer.cpp

Modified: lld/trunk/COFF/Writer.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.cpp?rev=366780&r1=366779&r2=366780&view=diff
==============================================================================
--- lld/trunk/COFF/Writer.cpp (original)
+++ lld/trunk/COFF/Writer.cpp Mon Jul 22 23:38:04 2019
@@ -762,6 +762,28 @@ void Writer::locateImportTables() {
   }
 }
 
+// Return whether a SectionChunk's suffix (the dollar and any trailing
+// suffix) should be removed and sorted into the main suffixless
+// PartialSection.
+static bool shouldStripSectionSuffix(SectionChunk *sc, StringRef name) {
+  // On MinGW, comdat groups are formed by putting the comdat group name
+  // after the '$' in the section name. For .eh_frame$<symbol>, that must
+  // still be sorted before the .eh_frame trailer from crtend.o, thus just
+  // strip the section name trailer. For other sections, such as
+  // .tls$$<symbol> (where non-comdat .tls symbols are otherwise stored in
+  // ".tls$"), they must be strictly sorted after .tls. And for the
+  // hypothetical case of comdat .CRT$XCU, we definitely need to keep the
+  // suffix for sorting. Thus, to play it safe, only strip the suffix for
+  // the standard sections.
+  if (!config->mingw)
+    return false;
+  if (!sc || !sc->isCOMDAT())
+    return false;
+  return name.startswith(".text$") || name.startswith(".data$") ||
+         name.startswith(".rdata$") || name.startswith(".pdata$") ||
+         name.startswith(".xdata$") || name.startswith(".eh_frame$");
+}
+
 // Create output section objects and add them to OutputSections.
 void Writer::createSections() {
   // First, create the builtin sections.
@@ -807,10 +829,7 @@ void Writer::createSections() {
       continue;
     }
     StringRef name = c->getSectionName();
-    // On MinGW, comdat groups are formed by putting the comdat group name
-    // after the '$' in the section name. Such a section name suffix shouldn't
-    // imply separate alphabetical sorting of those section chunks though.
-    if (config->mingw && sc && sc->isCOMDAT())
+    if (shouldStripSectionSuffix(sc, name))
       name = name.split('$').first;
     PartialSection *pSec = createPartialSection(name,
                                                 c->getOutputCharacteristics());

Added: lld/trunk/test/COFF/tls_suffix_sorting.s
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/COFF/tls_suffix_sorting.s?rev=366780&view=auto
==============================================================================
--- lld/trunk/test/COFF/tls_suffix_sorting.s (added)
+++ lld/trunk/test/COFF/tls_suffix_sorting.s Mon Jul 22 23:38:04 2019
@@ -0,0 +1,28 @@
+# REQUIRES: x86
+
+# RUN: echo -e ".section .tls,\"dw\"\n .byte 0xaa\n .section .tls\$ZZZ,\"dw\"\n .byte 0xff\n .globl _tls_index\n .data\n _tls_index:\n .int 0" > %t.tlssup.s
+# RUN: llvm-mc -triple=x86_64-windows-gnu %t.tlssup.s -filetype=obj -o %t.tlssup.o
+# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.main.o
+
+# RUN: lld-link -lldmingw -entry:main %t.main.o %t.tlssup.o -out:%t.exe
+# RUN: llvm-objdump -s %t.exe | FileCheck %s
+
+# Check that .tls$$foo is sorted after the start marker (aa) and before the
+# end marker (ff).
+
+# CHECK: Contents of section .tls:
+# CHECK:  140004000 aabbff
+
+        .text
+        .globl          main
+main:
+        movl            _tls_index(%rip), %eax
+        movq            %gs:88, %rcx
+        movq            (%rcx,%rax,8), %rax
+        movb            foo at SECREL32(%rax), %al
+        ret
+
+        .section        .tls$$foo,"dw"
+        .linkonce       discard
+foo:
+        .byte           0xbb