ELF: Discard .ARM.exidx sections for empty functions instead of misordering them.

The logic added in r372781 caused ARMExidxSyntheticSection::addSection()
to return false for exidx sections without a link order dep that passed
isValidExidxSectionDep(). This included exidx sections for empty functions. As
a result, such exidx sections would end up treated like ordinary sections and
would end up being laid out before the ARMExidxSyntheticSection, most likely in
the wrong order relative to the exidx entries in the ARMExidxSyntheticSection,
breaking the orderedness invariant relied upon by unwinders. Fix this by
simply discarding such sections.

Differential Revision: https://reviews.llvm.org/D69744

Author: pcc

lld/ELF/SyntheticSections.cpp

@@ -3164,12 +3164,10 @@ static bool isValidExidxSectionDep(InputSection *isec) {
 
 bool ARMExidxSyntheticSection::addSection(InputSection *isec) {
   if (isec->type == SHT_ARM_EXIDX) {
-    if (InputSection* dep = isec->getLinkOrderDep())
-      if (isValidExidxSectionDep(dep)) {
+    if (InputSection *dep = isec->getLinkOrderDep())
+      if (isValidExidxSectionDep(dep))
         exidxSections.push_back(isec);
-        return true;
-      }
-    return false;
+    return true;
   }
 
   if (isValidExidxSectionDep(isec)) {

lld/test/ELF/arm-exidx-empty-fn.s

@@ -0,0 +1,41 @@
+// REQUIRES: arm
+// RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+// RUN: ld.lld %t.o -o %t -shared
+
+// RUN: llvm-readelf --unwind %t | FileCheck %s
+
+// Check that any exidx sections for empty functions are discarded.
+
+// CHECK:      Entries [
+// CHECK-NEXT:   Entry {
+// CHECK-NEXT:     FunctionAddress:
+// CHECK-NEXT:     Model: CantUnwind
+// CHECK-NEXT:   }
+// CHECK-NEXT:   Entry {
+// CHECK-NEXT:     FunctionAddress:
+// CHECK-NEXT:     Model: CantUnwind
+// CHECK-NEXT:   }
+// CHECK-NEXT: ]
+
+.section .text.f0,"ax",%progbits
+.globl f0
+f0:
+.fnstart
+bx lr
+.cantunwind
+.fnend
+
+.section .text.f1,"ax",%progbits
+.globl f1
+f1:
+.fnstart
+.cantunwind
+.fnend
+
+.section .text.f2,"ax",%progbits
+.globl f2
+f2:
+.fnstart
+bx lr
+.cantunwind
+.fnend