[lld] fbf2f66 - [ELF] Update flag propagation rule to ignore discarded output sections

[Fangrui Song via llvm-commits]

Author: Fangrui Song
Date: 2022-02-01T10:19:30-08:00
New Revision: fbf2f6640067dec144f9aef8231d58c740e3d014

URL: https://github.com/llvm/llvm-project/commit/fbf2f6640067dec144f9aef8231d58c740e3d014
DIFF: https://github.com/llvm/llvm-project/commit/fbf2f6640067dec144f9aef8231d58c740e3d014.diff

LOG: [ELF] Update flag propagation rule to ignore discarded output sections

See the updated insert-before.test for the effects: many synthetic
sections are SHF_ALLOC|SHF_WRITE. If they are discarded, we don't want
to propagate their flags to subsequent output section descriptions.

`getFirstInputSection(sec) == nullptr` can technically be merged into
`isDiscardable` but I'd like to postpone that as not sharing code may give more
refactoring opportunity.

Depends on D118529.

Reviewed By: peter.smith, bluca

Differential Revision: https://reviews.llvm.org/D118530

Added: 
    

Modified: 
    lld/ELF/LinkerScript.cpp
    lld/test/ELF/linkerscript/insert-before.test

Removed: 
    


################################################################################
diff  --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index 7188ce58376d9..56d94744fae5e 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -1148,14 +1148,16 @@ void LinkerScript::adjustOutputSections() {
       sec->alignment =
           std::max<uint32_t>(sec->alignment, sec->alignExpr().getValue());
 
-    // The input section might have been removed (if it was an empty synthetic
-    // section), but we at least know the flags.
-    if (sec->hasInputSections)
+    bool isEmpty = (getFirstInputSection(sec) == nullptr);
+    bool discardable = isEmpty && isDiscardable(*sec);
+    // If sec has at least one input section and not discarded, remember its
+    // flags to be inherited by subsequent output sections. (sec may contain
+    // just one empty synthetic section.)
+    if (sec->hasInputSections && !discardable)
       flags = sec->flags;
 
     // We do not want to keep any special flags for output section
     // in case it is empty.
-    bool isEmpty = (getFirstInputSection(sec) == nullptr);
     if (isEmpty)
       sec->flags = flags & ((sec->nonAlloc ? 0 : (uint64_t)SHF_ALLOC) |
                             SHF_WRITE | SHF_EXECINSTR);
@@ -1172,7 +1174,7 @@ void LinkerScript::adjustOutputSections() {
     if (sec->sectionIndex != UINT32_MAX)
       maybePropagatePhdrs(*sec, defPhdrs);
 
-    if (isEmpty && isDiscardable(*sec)) {
+    if (discardable) {
       sec->markDead();
       cmd = nullptr;
     }

diff  --git a/lld/test/ELF/linkerscript/insert-before.test b/lld/test/ELF/linkerscript/insert-before.test
index f9611538c013b..922e0859106ff 100644
--- a/lld/test/ELF/linkerscript/insert-before.test
+++ b/lld/test/ELF/linkerscript/insert-before.test
@@ -29,7 +29,7 @@
 # CHECK2-NEXT:           NULL
 # CHECK2-NEXT: .foo.text PROGBITS 000000000020{{.*}} [[#%x,]] 000008 00  AX
 # CHECK2-NEXT: .text     PROGBITS [[#%x,]]           [[#%x,]] 000008 00  AX
-# CHECK2-NEXT: .byte     PROGBITS [[#%x,]]           [[#%x,]] 000001 00  WA
+# CHECK2-NEXT: .byte     PROGBITS [[#%x,]]           [[#%x,]] 000001 00  AX
 # CHECK2-NEXT: .foo.data PROGBITS [[#%x,]]           [[#%x,]] 000008 00  WA
 # CHECK2-NEXT: .data     PROGBITS [[#%x,]]           [[#%x,]] 000008 00  WA
 # CHECK2:      Type      {{.*}} Flg Align