[BOLT] Fix handling of ambiguous jump table entries (#155291)

Jump tables may contain entries that point immediately past the end of
their parent function. Normally, such entries are generated by the
compiler as a result of builtin_unreachable() case. We used to replace
those entries with a label belonging to their parent function assuming
the destination doesn't matter if it's an undefined behavior.

However, if such entry is at the end of the jump table, it could be a
real function pointer, not a jump table entry. We rely on heuristics to
detect such cases and can drop the trailing function pointer entries
from the table.

The problem presents when the "unreachable" ambiguous entry is followed
by another ambiguous entry corresponding to the start of the parent
function. In this case we accept pointers as entries and may incorrectly
update the function pointer.

The solution is to keep ambiguous "unreachable" jump table entries
identical to the original input, i.e. point to the same function. This
change does not affect CFG, but results in the entries being updated
with the new function address if it gets relocated.

Author: maksfb

bolt/lib/Core/BinaryBasicBlock.cpp

@@ -103,9 +103,18 @@ bool BinaryBasicBlock::validateSuccessorInvariants() {
         Valid &= (Sym == Function->getFunctionEndLabel() ||
                   Sym == Function->getFunctionEndLabel(getFragmentNum()));
         if (!Valid) {
-          BC.errs() << "BOLT-WARNING: Jump table contains illegal entry: "
-                    << Sym->getName() << "\n";
+          const BinaryFunction *TargetBF = BC.getFunctionForSymbol(Sym);
+          if (TargetBF) {
+            // It's possible for another function to be in the jump table entry
+            // as a result of built-in unreachable.
+            Valid = true;
+          } else {
+            BC.errs() << "BOLT-WARNING: Jump table contains illegal entry: "
+                      << Sym->getName() << "\n";
+          }
         }
+        if (!Valid)
+          break;
       }
     }
   } else {

bolt/lib/Core/BinaryFunction.cpp

@@ -1959,7 +1959,9 @@ void BinaryFunction::postProcessJumpTables() {
             return EntryAddress == Parent->getAddress() + Parent->getSize();
           });
       if (IsBuiltinUnreachable) {
-        MCSymbol *Label = getOrCreateLocalLabel(EntryAddress, true);
+        BinaryFunction *TargetBF = BC.getBinaryFunctionAtAddress(EntryAddress);
+        MCSymbol *Label = TargetBF ? TargetBF->getSymbol()
+                                   : getOrCreateLocalLabel(EntryAddress, true);
         JT.Entries.push_back(Label);
         continue;
       }

bolt/lib/Passes/IndirectCallPromotion.cpp

@@ -261,10 +261,7 @@ IndirectCallPromotion::getCallTargets(BinaryBasicBlock &BB,
     for (size_t I = Range.first; I < Range.second; ++I, JI += JIAdj) {
       MCSymbol *Entry = JT->Entries[I];
       const BinaryBasicBlock *ToBB = BF.getBasicBlockForLabel(Entry);
-      assert(ToBB || Entry == BF.getFunctionEndLabel() ||
-             Entry == BF.getFunctionEndLabel(FragmentNum::cold()));
-      if (Entry == BF.getFunctionEndLabel() ||
-          Entry == BF.getFunctionEndLabel(FragmentNum::cold()))
+      if (!ToBB)
         continue;
       const Location To(Entry);
       const BinaryBasicBlock::BinaryBranchInfo &BI = BB.getBranchInfo(*ToBB);

bolt/test/X86/jump-table-ambiguous-unreachable.s

@@ -0,0 +1,87 @@
+## Check that llvm-bolt correctly updates ambiguous jump table entries that
+## can correspond to either builtin_unreachable() or could be a pointer to
+## the next function.
+
+# REQUIRES: system-linux
+
+# RUN: llvm-mc -filetype=obj -triple x86_64-unknown-unknown %s -o %t.o
+# RUN: %clang %cflags %t.o -o %t.exe -no-pie -Wl,-q
+
+# RUN: llvm-bolt %t.exe --print-normalized --print-only=foo -o %t.out \
+# RUN:   2>&1 | FileCheck %s
+
+
+
+  .text
+  .globl _start
+  .type _start, %function
+_start:
+  .cfi_startproc
+  call foo
+  ret
+  .cfi_endproc
+  .size _start, .-_start
+
+  .globl foo
+  .type foo, %function
+foo:
+	.cfi_startproc
+.LBB00:
+          movq	0x8(%rdi), %rdi
+          movzbl	0x1(%rdi), %eax
+.LBB00_br:
+	        jmpq	*"JUMP_TABLE/foo.0"(,%rax,8)
+# CHECK:  jmpq {{.*}} # JUMPTABLE
+# CHECK-NEXT: Successors: {{.*}}, {{.*}}, {{.*}}, {{.*}}, {{.*}}
+
+.Ltmp87085:
+	xorl	%eax, %eax
+	retq
+
+.Ltmp87086:
+	cmpb	$0x0, 0x8(%rdi)
+	setne	%al
+	retq
+
+.Ltmp87088:
+	movb	$0x1, %al
+	retq
+
+.Ltmp87087:
+	movzbl	0x14(%rdi), %eax
+	andb	$0x2, %al
+	shrb	%al
+	retq
+
+	.cfi_endproc
+.size foo, .-foo
+
+  .globl bar
+  .type bar, %function
+bar:
+	.cfi_startproc
+  ret
+	.cfi_endproc
+  .size bar, .-bar
+
+# Jump tables
+.section .rodata
+  .global jump_table
+jump_table:
+"JUMP_TABLE/foo.0":
+  .quad bar
+  .quad	.Ltmp87085
+  .quad bar
+  .quad	.Ltmp87086
+  .quad	.Ltmp87087
+  .quad	.LBB00
+  .quad	.Ltmp87088
+  .quad bar
+  .quad	.LBB00
+
+# CHECK: Jump table {{.*}} for function foo
+# CHECK-NEXT: 0x{{.*}} : bar
+# CHECK-NEXT: 0x{{.*}} :
+# CHECK-NEXT: 0x{{.*}} : bar
+# CHECK-NEXT: 0x{{.*}} :
+# CHECK-NEXT: 0x{{.*}} :