[PATCH] D49622: ELF: Make sections with KeepUnique bit eligible for ICF.
Peter Collingbourne via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Fri Jul 20 17:22:26 PDT 2018
This revision was automatically updated to reflect the committed changes.
Closed by commit rLLD337638: ELF: Make sections with KeepUnique bit eligible for ICF. (authored by pcc, committed by ).
Changed prior to commit:
https://reviews.llvm.org/D49622?vs=156649&id=156651#toc
Repository:
rLLD LLVM Linker
https://reviews.llvm.org/D49622
Files:
ELF/ICF.cpp
test/ELF/icf-keep-unique.s
Index: test/ELF/icf-keep-unique.s
===================================================================
--- test/ELF/icf-keep-unique.s
+++ test/ELF/icf-keep-unique.s
@@ -11,10 +11,13 @@
// CHECK-NEXT: removing identical section {{.*}}:(.text.f4)
// CHECK-NEXT: removing identical section {{.*}}:(.text.f5)
-// With --keep-unique f2, f4 and f5 we expect only f3 and f5 to be removed.
+// With --keep-unique f2, f4 and f5 we expect only f2, f3 and f5 to be removed.
+// f2 can be removed despite being --keep-unique because all sections that are
+// merged into it are not --keep-unique.
// f5 is not matched by --keep-unique as it is a local symbol.
// CHECK-KEEP: warning: could not find symbol f5 to keep unique
// CHECK-KEEP: selected section {{.*}}:(.text.f1)
+// CHECK-KEEP-NEXT: removing identical section {{.*}}:(.text.f2)
// CHECK-KEEP-NEXT: removing identical section {{.*}}:(.text.f3)
// CHECK-KEEP-NEXT: removing identical section {{.*}}:(.text.f5)
.globl _start, f1, f2, f3, f4
Index: ELF/ICF.cpp
===================================================================
--- ELF/ICF.cpp
+++ ELF/ICF.cpp
@@ -163,7 +163,7 @@
// Returns true if section S is subject of ICF.
static bool isEligible(InputSection *S) {
- if (!S->Live || S->KeepUnique || !(S->Flags & SHF_ALLOC))
+ if (!S->Live || !(S->Flags & SHF_ALLOC))
return false;
// Don't merge writable sections. .data.rel.ro sections are marked as writable
@@ -468,10 +468,29 @@
forEachClassRange(0, Sections.size(), [&](size_t Begin, size_t End) {
if (End - Begin == 1)
return;
- print("selected section " + toString(Sections[Begin]));
- for (size_t I = Begin + 1; I < End; ++I) {
+ InputSection *Target = nullptr;
+ bool SeenUnique = false, Replaced = false;
+ for (size_t I = Begin; I < End; ++I) {
+ // We aren't allowed to merge two KeepUnique sections as this would break
+ // the guarantees provided by --keep-unique and --icf=safe, but there's
+ // no reason why we can't merge a non-KeepUnique section with a KeepUnique
+ // section. We implement this by only considering the first KeepUnique
+ // section in the equivalence class for merging. If we see any more
+ // KeepUnique sections, we ignore them.
+ if (Sections[I]->KeepUnique) {
+ if (SeenUnique)
+ continue;
+ SeenUnique = true;
+ }
+ if (!Target) {
+ Target = Sections[I];
+ continue;
+ }
+ if (!Replaced)
+ print("selected section " + toString(Target));
print(" removing identical section " + toString(Sections[I]));
- Sections[Begin]->replace(Sections[I]);
+ Target->replace(Sections[I]);
+ Replaced = true;
// At this point we know sections merged are fully identical and hence
// we want to remove duplicate implicit dependencies such as link order
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