[ELF] Allow absolute relocation referencing symbol index 0 in PIC mode

The value of an absolute relocation, like R_RISCV_HI20 or R_PPC64_LO16,
with a symbol index of 0, the resulting value should be treated as
absolute and permitted in both -pie and -shared links.

This change also resolves an absolute relocation referencing an
undefined symbol in statically-linked executables.

PPC64 has unfortunate exceptions:

* R_PPC64_TOCBASE uses symbol index 0 but it should be treated as
  referencing the linker-defined .TOC.
* R_PPC64_PCREL_OPT (https://reviews.llvm.org/D84360) could no longer
  rely on `isAbsoluteValue` return false.

Author: MaskRay

lld/ELF/Relocations.cpp

@@ -178,7 +178,7 @@ static RelType getMipsPairType(RelType type, bool isLocal) {
 // True if non-preemptable symbol always has the same value regardless of where
 // the DSO is loaded.
 static bool isAbsolute(const Symbol &sym) {
-  if (sym.isUndefWeak())
+  if (sym.isUndefined())
     return true;
   if (const auto *dr = dyn_cast<Defined>(&sym))
     return dr->section == nullptr; // Absolute symbol.
@@ -1005,7 +1005,7 @@ bool RelocationScanner::isStaticLinkTimeConstant(RelExpr e, RelType type,
 
   // For the target and the relocation, we want to know if they are
   // absolute or relative.
-  bool absVal = isAbsoluteValue(sym);
+  bool absVal = isAbsoluteValue(sym) && e != RE_PPC64_TOCBASE;
   bool relE = isRelExpr(e);
   if (absVal && !relE)
     return true;
@@ -1021,7 +1021,7 @@ bool RelocationScanner::isStaticLinkTimeConstant(RelExpr e, RelType type,
   // calls to such symbols (e.g. glibc/stdlib/exit.c:__run_exit_handlers).
   // Normally such a call will be guarded with a comparison, which will load a
   // zero from the GOT.
-  if (sym.isUndefWeak())
+  if (sym.isUndefined())
     return true;
 
   // We set the final symbols values for linker script defined symbols later.
@@ -1066,7 +1066,8 @@ void RelocationScanner::processAux(RelExpr expr, RelType type, uint64_t offset,
              type == R_HEX_GD_PLT_B22_PCREL_X ||
              type == R_HEX_GD_PLT_B32_PCREL_X)))
         expr = fromPlt(expr);
-    } else if (!isAbsoluteValue(sym)) {
+    } else if (!isAbsoluteValue(sym) ||
+               (type == R_PPC64_PCREL_OPT && ctx.arg.emachine == EM_PPC64)) {
       expr = ctx.target->adjustGotPcExpr(type, addend,
                                          sec->content().data() + offset);
       // If the target adjusted the expression to R_RELAX_GOT_PC, we may end up

lld/test/ELF/riscv-relax-hi20-lo12-pie.s

@@ -12,6 +12,8 @@
 
 # CHECK:      lui     a0, 0x200
 # CHECK-NEXT: addi    a0, a0, 0x1
+# CHECK-NEXT: lui     a0, 0x200
+# CHECK-NEXT: addi    a0, a0, 0x1
 # CHECK-NEXT: lw      a0, 0x1(a0)
 # CHECK-NEXT: sw      a0, 0x1(a0)
 
@@ -23,6 +25,11 @@ abs = 0x200001
 _start:
   lui a0, %hi(abs)
   addi a0, a0, %lo(abs)
+  .reloc ., R_RISCV_HI20, abs
+  lui a0, 0
+  .reloc ., R_RISCV_LO12_I, abs
+  addi a0, a0, 0
+
   lw a0, %lo(abs)(a0)
   sw a0, %lo(abs)(a0)
 

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

@@ -29,10 +29,12 @@
 # RUN: ld.lld a.o b.o -o out1 -z undefs
 # RUN: llvm-readelf -r -x .data out1 | FileCheck %s --check-prefix=STATIC1
 # RUN: ld.lld a.o b.o -o out1.pie -pie -z undefs
-# RUN: llvm-readelf -r -x .data out1.pie | FileCheck %s --check-prefix=STATIC2
+# RUN: llvm-readelf -r -x .data out1.pie | FileCheck %s --check-prefix=STATIC1
 
 # STATIC1:      no relocations
-# STATIC2:      R_X86_64_RELATIVE
+# STATIC1:      Hex dump of section '.data':
+# STATIC1-NEXT: {{.*}} 00000000 00000000 00000000 00000000 .
+# STATIC1-EMPTY:
 
 # RUN: ld.lld a.o b.o c.o -pie -z undefs 2>&1 | count 0