[BOLT] Prevent adding secondary entry points for BB labels

[jobnoorman]

Summary

When linker relaxation is enabled on RISC-V, every branch has a relocation and a
corresponding symbol in the symbol table. BOLT currently registers all these
symbols as secondary entry points causing almost every function to be marked as
multi entry on RISC-V.

This patch modifies adjustFunctionBoundaries to ignore these symbols.
Note that I currently try to detect them by checking if a symbol's name
starts with the private label prefix as defined by MCAsmInfo. Since
I'm not entirely sure what multi-entry functions look like on different
targets, please check if this condition is correct. Maybe it could make
sense to only check this on RISC-V?

[maksfb]

bolt/lib/Rewrite/RewriteInstance.cpp

1592–1593

It makes sense to run this check for other platforms as well, like it's written. In that case, remove "[RISCV]" from the title. Just to be safe, add a check that the prefix is not empty.

[jobnoorman]

• Check that prefix is not empty
• Remove [RISCV] from title

bolt/lib/Rewrite/RewriteInstance.cpp

while (NextSymRefI != FileSymRefs.end()) {
       const uint64_t SymbolSize = ELFSymbolRef(Symbol).getSize();
 
       if (NextFunction && SymbolAddress >= NextFunction->getAddress())
         break;
 
       if (!Function.isSymbolValidInScope(Symbol, SymbolSize))
         break;
 
+      // Skip basic block labels. This happens on RISC-V with linker relaxation
+      // enabled because every branch needs a relocation and corresponding
+      // symbol. We don't want to add such symbols as entry points.
+      const auto PrivateLabelPrefix = BC->AsmInfo->getPrivateLabelPrefix();
+      if (!PrivateLabelPrefix.empty() &&
+          cantFail(Symbol.getName()).starts_with(PrivateLabelPrefix)) {
+        ++NextSymRefI;
+        continue;
+      }
+
       // This is potentially another entry point into the function.
       uint64_t EntryOffset = NextSymRefI->first - Function.getAddress();
       LLVM_DEBUG(dbgs() << "BOLT-DEBUG: adding entry point to function "
                         << Function << " at offset 0x"
                         << Twine::utohexstr(EntryOffset) << '\n');
       Function.addEntryPointAtOffset(EntryOffset);
 
       ++NextSymRefI;

bolt/test/RISCV/branch-no-secondary-entry.s

+/// Test that no secondary entry points are created for basic block labels used
+/// by branches.
+// RUN: %clang %cflags -o %t %s
+// RUN: llvm-bolt -print-cfg -o /dev/null %t 2>&1 | FileCheck %s
+
+// CHECK: Binary Function "_start" after building cfg {
+// CHECK: IsMultiEntry: 0
+// CHECK: beq t0, t1, .Ltmp0
+// CHECK: {{^}}.Ltmp0
+// CHECK: ret
+
+    .globl _start
+_start:
+    beq t0, t1, 1f
+1:
+    ret
+    .size _start, .-_start
+