[ELF] - Do not crash when discarding sections that are referenced by others.

[grimar]

Summary

SHF_LINK_ORDER sections adds special ordering requirements.
Such sections references other sections. Previously we would crash
if section that other were referenced to was discarded by script.

Patch fixes that by discarding all dependent sections in that case.
It supports chained dependencies, testcase is provided.

[ruiu]

lld/trunk/ELF/LinkerScript.cpp

282 ↗

(On Diff #88739)

You don't need to check if a container is empty before passing it to a for-each-ish function, just like you don't check for emptiness foo before doing for (auto bar : foo).

[grimar]

lld/trunk/ELF/LinkerScript.cpp

282 ↗

(On Diff #88739)

I know. And even applied that for simplicity in D29273.
But there I do not have cast. Since in this patch I had to do cast and anyways had to check !IS condition, I see nothing wrong in checking empty container to exit early. Code would not became simpler here, early continue still simplifies reading.

ELF/LinkerScript.cpp

void LinkerScript<ELFT>::computeInputSections(InputSectionDescription *I) {
   }
 }
 
 template <class ELFT>
 void LinkerScript<ELFT>::discard(ArrayRef<InputSectionBase<ELFT> *> V) {
   for (InputSectionBase<ELFT> *S : V) {
     S->Live = false;
     reportDiscarded(S);
+
+    InputSection<ELFT> *IS = dyn_cast<InputSection<ELFT>>(S);
+    if (!IS || IS->DependentSections.empty())
+      continue;
+    discard(IS->DependentSections);
   }
 }
 
 template <class ELFT>
 std::vector<InputSectionBase<ELFT> *>
 LinkerScript<ELFT>::createInputSectionList(OutputSectionCommand &OutCmd) {
   std::vector<InputSectionBase<ELFT> *> Ret;
 

test/ELF/linkerscript/discard-section-metadata.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+# RUN: echo "SECTIONS { /DISCARD/ : { *(.foo) } }" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -section-headers %t1 | FileCheck %s
+
+# CHECK-NOT: .foo
+# CHECK-NOT: .bar
+# CHECK-NOT: .zed
+# CHECK-NOT: .moo
+
+## Sections dependency tree for testcase is:
+## (.foo)
+##   | |
+##   | --(.bar)
+##   |
+##   --(.zed)
+##       |
+##       --(.moo)
+##
+
+.section .foo,"a"
+.quad 0
+
+.section .bar,"am",@progbits,.foo
+.quad 0
+
+.section .zed,"am",@progbits,.foo
+.quad 0
+
+.section .moo,"am",@progbits,.zed
+.quad 0