[cfi] Build __cfi_check as Thumb when applicable.

[eugenis]

Summary

Cross-DSO CFI needs all __cfi_check exports to use the same encoding
(ARM vs Thumb).

[pcc]

LGTM

llvm/test/Transforms/CrossDSOCFI/thumb.ll

2

Maybe add a negative test?

[eugenis]

add a negative test

[echristo]

Not the way I'd have done this. What you probably need to do if adding a stub is propagate the features from the function you're cloning rather than constructing a new (and possibly incomplete) set of features from the target triple yourself.

[eugenis]

There is nothing to propagate from: it's a new function, not a clone of any other.
I guess we could ask the frontend to emit a default set of target features?

[echristo]

Which isn't necessarily going to be valid either - particularly in the face of LTO.

What are you trying to accomplish here?

[eugenis]

__cfi_check is a compiler-generated function (frontend inserts a stub, then LTO fills in the body). I want it to be built as Thumb, and never as ARM.

[echristo]

Have the front end generate all of the correct target features for your stub then rather than adding them in the backend?

[eugenis]

Sounds good, I'll do that.

llvm/lib/Transforms/IPO/CrossDSOCFI.cpp

 // __cfi_check function, which can be used to verify cross-DSO call targets.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Transforms/IPO/CrossDSOCFI.h"
 #include "llvm/ADT/DenseSet.h"
 #include "llvm/ADT/EquivalenceClasses.h"
 #include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/Triple.h"
 #include "llvm/IR/Constant.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Function.h"
 #include "llvm/IR/GlobalObject.h"
 #include "llvm/IR/GlobalVariable.h"
 #include "llvm/IR/IRBuilder.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Intrinsics.h"
void CrossDSOCFI::buildCFICheck(Module &M) {
   Constant *C = M.getOrInsertFunction(
       "__cfi_check", Type::getVoidTy(Ctx), Type::getInt64Ty(Ctx),
       Type::getInt8PtrTy(Ctx), Type::getInt8PtrTy(Ctx));
   Function *F = dyn_cast<Function>(C);
   // Take over the existing function. The frontend emits a weak stub so that the
   // linker knows about the symbol; this pass replaces the function body.
   F->deleteBody();
   F->setAlignment(4096);
+
+  Triple T(M.getTargetTriple());
+  if (T.isARM() || T.isThumb())
+    F->addFnAttr("target-features", "+thumb-mode");
+
   auto args = F->arg_begin();
   Value &CallSiteTypeId = *(args++);
   CallSiteTypeId.setName("CallSiteTypeId");
   Value &Addr = *(args++);
   Addr.setName("Addr");
   Value &CFICheckFailData = *(args++);
   CFICheckFailData.setName("CFICheckFailData");
   assert(args == F->arg_end());

llvm/test/Transforms/CrossDSOCFI/thumb.ll

+; RUN: opt -mtriple=armv7-linux-android -S -cross-dso-cfi < %s | FileCheck %s
+; RUN: opt -mtriple=thumbv7-linux-android -S -cross-dso-cfi < %s | FileCheck %s
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+
+define signext i8 @f() !type !0 !type !1 {
+entry:
+  ret i8 1
+}
+
+!llvm.module.flags = !{!2}
+
+!0 = !{i64 0, !"_ZTSFcvE"}
+!1 = !{i64 0, i64 111}
+!2 = !{i32 4, !"Cross-DSO CFI", i32 1}
+
+; CHECK: define void @__cfi_check({{.*}} #[[A:.*]] align 4096
+; CHECK: attributes #[[A]] = { {{.*}}"target-features"="+thumb-mode"