From 23ec5782c3cc0d77b5e44285e5124cd4a3ffeeef Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers@google.com>
Date: Tue, 12 Apr 2022 11:37:42 -0700
Subject: [Bitcode] materialize Functions early when BlockAddress taken

IRLinker builds a work list of functions to materialize, then moves them
from a source module to a destination module one at a time.

This is a problem for blockaddress Constants, since they need not refer
to the function they are used in; IPSCCP is quite good at sinking these
constants deep into other functions when passed as arguments.

This would lead to curious errors during LTO:
  ld.lld: error: Never resolved function from blockaddress ...
based on the ordering of function definitions in IR.

The problem was that IRLinker would basically do:

  for function f in worklist:
    materialize f
    splice f from source module to destination module

in one pass, with Functions being lazily added to the running worklist.
This confuses BitcodeReader, which cannot disambiguate whether a
blockaddress is referring to a function which has not yet been parsed
("materialized") or is simply empty because its body was spliced out.
This causes BitcodeReader to insert Functions into its BasicBlockFwdRefs
list incorrectly, as it will never re-materialize an already
materialized (but spliced out) function.

Because of the possibility that blockaddress Constants may appear in
Functions other than the ones they reference, this patch adds a new
bitcode function code FUNC_CODE_BLOCKADDR_USERS that is a simple list of
Functions that contain BlockAddress Constants that refer back to this
Function, rather then the Function they are scoped in. We then
materialize those functions when materializing `f` from the example loop
above. This might over-materialize Functions should the user of
BitcodeReader ultimately decide not to link those Functions, but we can
at least now we can avoid this ordering related issue with blockaddresses.

Fixes: https://github.com/llvm/llvm-project/issues/52787
Fixes: https://github.com/ClangBuiltLinux/linux/issues/1215

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D120781
---
 llvm/lib/Bitcode/Writer/BitcodeWriter.cpp | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

(limited to 'llvm/lib/Bitcode/Writer/BitcodeWriter.cpp')

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index c76294d..79c7685 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -19,6 +19,7 @@
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/Optional.h"
 #include "llvm/ADT/STLExtras.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallString.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringMap.h"
@@ -3359,8 +3360,10 @@ void ModuleBitcodeWriter::writeFunction(
   bool NeedsMetadataAttachment = F.hasMetadata();
 
   DILocation *LastDL = nullptr;
+  SmallPtrSet<Function *, 4> BlockAddressUsers;
+
   // Finally, emit all the instructions, in order.
-  for (const BasicBlock &BB : F)
+  for (const BasicBlock &BB : F) {
     for (const Instruction &I : BB) {
       writeInstruction(I, InstID, Vals);
 
@@ -3392,6 +3395,25 @@ void ModuleBitcodeWriter::writeFunction(
       LastDL = DL;
     }
 
+    if (BlockAddress *BA = BlockAddress::lookup(&BB)) {
+      for (User *U : BA->users()) {
+        if (auto *I = dyn_cast<Instruction>(U)) {
+          Function *P = I->getParent()->getParent();
+          if (P != &F)
+            BlockAddressUsers.insert(P);
+        }
+      }
+    }
+  }
+
+  if (!BlockAddressUsers.empty()) {
+    SmallVector<uint64_t, 4> Record;
+    Record.reserve(BlockAddressUsers.size());
+    for (Function *F : BlockAddressUsers)
+      Record.push_back(VE.getValueID(F));
+    Stream.EmitRecord(bitc::FUNC_CODE_BLOCKADDR_USERS, Record);
+  }
+
   // Emit names for all the instructions etc.
   if (auto *Symtab = F.getValueSymbolTable())
     writeFunctionLevelValueSymbolTable(*Symtab);
-- 
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