Support for taking the max of module flags when linking, use for PIE/PIC

Summary:
Add Max ModFlagBehavior, which can be used to take the max of two
module flag values when merging modules. Use it for the PIE and PIC
levels.

This avoids an error when we try to import from a module built -fpic
into a module built -fPIC, for example. For both PIE and PIC levels,
this will be legal, since the code generation gets more conservative
as the level is increased. Therefore we can take the max instead of
somehow trying to block importing between modules compiled with
different levels.

Reviewers: tmsriram, pcc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33418

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303590 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 15 insertions, 2 deletions

diff --git a/lib/Linker/IRMover.cpp b/lib/Linker/IRMover.cpp
index c0af21aa148..defad190498 100644
--- a/lib/Linker/IRMover.cpp
+++ b/lib/Linker/IRMover.cpp
@@ -1157,6 +1157,11 @@ Error IRLinker::linkModuleFlagsMetadata() {
         mdconst::extract<ConstantInt>(DstOp->getOperand(0));
     unsigned DstBehaviorValue = DstBehavior->getZExtValue();
 
+    auto overrideDstValue = [&]() {
+      DstModFlags->setOperand(DstIndex, SrcOp);
+      Flags[ID].first = SrcOp;
+    };
+
     // If either flag has override behavior, handle it first.
     if (DstBehaviorValue == Module::Override) {
       // Diagnose inconsistent flags which both have override behavior.
@@ -1167,8 +1172,7 @@ Error IRLinker::linkModuleFlagsMetadata() {
       continue;
     } else if (SrcBehaviorValue == Module::Override) {
       // Update the destination flag to that of the source.
-      DstModFlags->setOperand(DstIndex, SrcOp);
-      Flags[ID].first = SrcOp;
+      overrideDstValue();
       continue;
     }
 
@@ -1204,6 +1208,15 @@ Error IRLinker::linkModuleFlagsMetadata() {
       }
       continue;
     }
+    case Module::Max: {
+      ConstantInt *DstValue =
+          mdconst::extract<ConstantInt>(DstOp->getOperand(2));
+      ConstantInt *SrcValue =
+          mdconst::extract<ConstantInt>(SrcOp->getOperand(2));
+      if (SrcValue->getZExtValue() > DstValue->getZExtValue())
+        overrideDstValue();
+      break;
+    }
     case Module::Append: {
       MDNode *DstValue = cast<MDNode>(DstOp->getOperand(2));
       MDNode *SrcValue = cast<MDNode>(SrcOp->getOperand(2));