[GlobalMerge] Don't merge globals that may be preempted

[john.brawn]

Summary

When a global may be preempted it needs to be accessed directly, instead of indirectly through a MergedGlobals symbol, for the preemption to work.

This fixes PR33136.

[peter.smith]

Change looks sensible to me. I'll rebuild and run the libc++ tests on ARM again and report back.

[peter.smith]

A Release Build of clang with that change has passed libc++ and libc++abi tests. I think that this is good to go, although it will be worth waiting to see if there are any objections from a later timezone first.

llvm/trunk/lib/CodeGen/GlobalMerge.cpp

bool GlobalMerge::doInitialization(Module &M) {
   setMustKeepGlobalVariables(M);
 
   // Grab all non-const globals.
   for (auto &GV : M.globals()) {
     // Merge is safe for "normal" internal or external globals only
     if (GV.isDeclaration() || GV.isThreadLocal() || GV.hasSection())
       continue;
 
+    // It's not safe to merge globals that may be preempted
+    if (TM && !TM->shouldAssumeDSOLocal(M, &GV))
+      continue;
+
     if (!(MergeExternalGlobals && GV.hasExternalLinkage()) &&
         !GV.hasInternalLinkage())
       continue;
 
     PointerType *PT = dyn_cast<PointerType>(GV.getType());
     assert(PT && "Global variable is not a pointer!");
 
     unsigned AddressSpace = PT->getAddressSpace();

llvm/trunk/test/CodeGen/ARM/global-merge-external.ll

 ; RUN: llc < %s -mtriple=arm-eabi  -arm-global-merge                                 | FileCheck %s --check-prefix=CHECK-MERGE
 ; RUN: llc < %s -mtriple=arm-eabi  -arm-global-merge -global-merge-on-external=true  | FileCheck %s --check-prefix=CHECK-MERGE
 ; RUN: llc < %s -mtriple=arm-eabi  -arm-global-merge -global-merge-on-external=false | FileCheck %s --check-prefix=CHECK-NO-MERGE
 ; RUN: llc < %s -mtriple=arm-macho -arm-global-merge                                 | FileCheck %s --check-prefix=CHECK-NO-MERGE
+; RUN: llc < %s -mtriple=arm-eabi  -arm-global-merge -relocation-model=pic           | FileCheck %s --check-prefix=CHECK-NO-MERGE
 
 @x = global i32 0, align 4
 @y = global i32 0, align 4
 @z = global i32 0, align 4
 
 define void @f1(i32 %a1, i32 %a2) {
 ;CHECK:          f1:
 ;CHECK:          ldr {{r[0-9]+}}, [[LABEL1:\.LCPI[0-9]+_[0-9]]]