[BOLT] Accept PLT fall-throughs as valid traces (#129481)

We used to report PLT traces as invalid (mismatching disassembled
function contents) because PLT functions are marked as pseudo and
ignored, thus missing CFG. However, such traces are not mismatching
the function contents. Accept them without attaching the profile.

Test Plan: updated callcont-fallthru.s

Author: aaupov

bolt/lib/Profile/DataAggregator.cpp

@@ -871,18 +871,27 @@ DataAggregator::getFallthroughsInTrace(BinaryFunction &BF,
 
   BinaryContext &BC = BF.getBinaryContext();
 
-  if (!BF.isSimple())
-    return std::nullopt;
-
-  assert(BF.hasCFG() && "can only record traces in CFG state");
-
   // Offsets of the trace within this function.
   const uint64_t From = FirstLBR.To - BF.getAddress();
   const uint64_t To = SecondLBR.From - BF.getAddress();
 
   if (From > To)
     return std::nullopt;
 
+  // Accept fall-throughs inside pseudo functions (PLT/thunks).
+  // This check has to be above BF.empty as pseudo functions would pass it:
+  // pseudo => ignored => CFG not built => empty.
+  // If we return nullopt, trace would be reported as mismatching disassembled
+  // function contents which it is not. To avoid this, return an empty
+  // fall-through list instead.
+  if (BF.isPseudo())
+    return Branches;
+
+  if (!BF.isSimple())
+    return std::nullopt;
+
+  assert(BF.hasCFG() && "can only record traces in CFG state");
+
   const BinaryBasicBlock *FromBB = BF.getBasicBlockContainingOffset(From);
   const BinaryBasicBlock *ToBB = BF.getBasicBlockContainingOffset(To);
 

bolt/test/X86/callcont-fallthru.s

@@ -9,6 +9,7 @@
 # RUN: link_fdata %s %t %t.pa3 PREAGG3
 # RUN: link_fdata %s %t %t.pat PREAGGT1
 # RUN: link_fdata %s %t %t.pat2 PREAGGT2
+# RUN: link_fdata %s %t %t.patplt PREAGGPLT
 
 ## Check normal case: fallthrough is not LP or secondary entry.
 # RUN: llvm-strip --strip-unneeded %t -o %t.strip
@@ -42,6 +43,12 @@
 # RUN: llvm-bolt %t.strip --pa -p %t.pat2 -o %t.out \
 # RUN:   --print-cfg --print-only=main | FileCheck %s --check-prefix=CHECK3
 
+## Check pre-aggregated traces don't report zero-sized PLT fall-through as
+## invalid trace
+# RUN: llvm-bolt %t.strip --pa -p %t.patplt -o %t.out | FileCheck %s \
+# RUN:   --check-prefix=CHECK-PLT
+# CHECK-PLT: traces mismatching disassembled function contents: 0
+
   .globl foo
   .type foo, %function
 foo:
@@ -65,7 +72,10 @@ main:
 	movl	$0x0, -0x4(%rbp)
 	movl	%edi, -0x8(%rbp)
 	movq	%rsi, -0x10(%rbp)
+Ltmp0_br:
 	callq	puts@PLT
+## Check PLT traces are accepted
+# PREAGGPLT: T #Ltmp0_br# #puts@plt# #puts@plt# 3
 ## Target is an external-origin call continuation
 # PREAGG1: B X:0 #Ltmp1# 2 0
 # PREAGGT1: T X:0 #Ltmp1# #Ltmp4_br# 2

bolt/test/link_fdata.py

@@ -8,6 +8,7 @@
 """
 
 import argparse
+import os
 import subprocess
 import sys
 import re
@@ -84,8 +85,16 @@
             exit("ERROR: unexpected input:\n%s" % line)
 
 # Read nm output: <symbol value> <symbol type> <symbol name>
+is_llvm_nm = os.path.basename(args.nmtool) == "llvm-nm"
 nm_output = subprocess.run(
-    [args.nmtool, "--defined-only", args.objfile], text=True, capture_output=True
+    [
+        args.nmtool,
+        "--defined-only",
+        "--special-syms" if is_llvm_nm else "--synthetic",
+        args.objfile,
+    ],
+    text=True,
+    capture_output=True,
 ).stdout
 # Populate symbol map
 symbols = {}