[ELF] -z undefs: handle relocations referencing undefined non-weak like undefined weak

* Merge the special case into isStaticLinkTimeConstant
* Generalize isUndefWeak to isUndefined. undefined non-weak is an error
  case. We choose to be general, which also brings us in line with GNU ld.

Author: MaskRay

lld/ELF/Relocations.cpp

@@ -990,10 +990,17 @@ bool RelocationScanner::isStaticLinkTimeConstant(RelExpr e, RelType type,
   // only the low bits are used.
   if (e == R_GOT || e == R_PLT)
     return ctx.target->usesOnlyLowPageBits(type) || !ctx.arg.isPic;
-
   // R_AARCH64_AUTH_ABS64 requires a dynamic relocation.
-  if (sym.isPreemptible || e == RE_AARCH64_AUTH)
+  if (e == RE_AARCH64_AUTH)
     return false;
+
+  // The behavior of an undefined weak reference is implementation defined.
+  // (We treat undefined non-weak the same as undefined weak.) For static
+  // -no-pie linking, dynamic relocations are generally avoided (except
+  // IRELATIVE). Emitting dynamic relocations for -shared aligns with its -z
+  // undefs default. Dynamic -no-pie linking and -pie allow flexibility.
+  if (sym.isPreemptible)
+    return sym.isUndefined() && !ctx.arg.isPic;
   if (!ctx.arg.isPic)
     return true;
 
@@ -1113,19 +1120,7 @@ void RelocationScanner::processAux(RelExpr expr, RelType type, uint64_t offset,
   // If the relocation is known to be a link-time constant, we know no dynamic
   // relocation will be created, pass the control to relocateAlloc() or
   // relocateNonAlloc() to resolve it.
-  //
-  // The behavior of an undefined weak reference is implementation defined. For
-  // non-link-time constants, we resolve relocations statically (let
-  // relocate{,Non}Alloc() resolve them) for -no-pie and try producing dynamic
-  // relocations for -pie and -shared.
-  //
-  // The general expectation of -no-pie static linking is that there is no
-  // dynamic relocation (except IRELATIVE). Emitting dynamic relocations for
-  // -shared matches the spirit of its -z undefs default. -pie has freedom on
-  // choices, and we choose dynamic relocations to be consistent with the
-  // handling of GOT-generating relocations.
-  if (isStaticLinkTimeConstant(expr, type, sym, offset) ||
-      (!ctx.arg.isPic && sym.isUndefWeak())) {
+  if (isStaticLinkTimeConstant(expr, type, sym, offset)) {
     sec->addReloc({expr, type, offset, addend, &sym});
     return;
   }

lld/test/ELF/weak-undef-rw.s

@@ -33,19 +33,21 @@
 # RUN: ld.lld a.o b.o -o ab -z undefs
 # RUN: llvm-readelf -r -x .data ab | FileCheck %s --check-prefix=STATIC1
 # RUN: ld.lld a.o b.o s.so -o abs -z undefs
-# RUN: llvm-readelf -r -x .data abs | FileCheck %s --check-prefix=DYN1
-# RUN: ld.lld a.o b.o -o abs.pie -pie -z undefs
-# RUN: llvm-readelf -r -x .data abs.pie | FileCheck %s --check-prefix=STATIC1
+# RUN: llvm-readelf -r -x .data abs | FileCheck %s --check-prefix=STATIC1
+# RUN: ld.lld a.o b.o -o ab.pie -pie -z undefs
+# RUN: llvm-readelf -r -x .data ab.pie | FileCheck %s --check-prefix=STATIC1
+# RUN: ld.lld a.o b.o s.so -o abs.pie -pie -z undefs
+# RUN: llvm-readelf -r -x .data abs.pie | FileCheck %s --check-prefix=DYN1
 
 # STATIC1:      no relocations
 # STATIC1:      Hex dump of section '.data':
 # STATIC1-NEXT: {{.*}} 00000000 00000000 03000000 00000000 .
 # STATIC1-NEXT: {{.*}} 05000000 00000000                   .
 # STATIC1-EMPTY:
 
-# DYN1:        Relocation section '.rela.dyn' {{.*}} contains 1
+# DYN1:        Relocation section '.rela.dyn' {{.*}} contains 3
 # DYN1:        Hex dump of section '.data':
-# DYN1-NEXT:   {{.*}} 00000000 00000000 03000000 00000000 .
+# DYN1-NEXT:   {{.*}} 00000000 00000000 00000000 00000000 .
 # DYN1-NEXT:   {{.*}} 00000000 00000000                   .
 # DYN1-EMPTY: