Pull in r257902 from upstream llvm trunk, by James Y Knight (this will

be merged to the official release_38 branch soon, but we need it ASAP):

  Stop increasing alignment of externally-visible globals on ELF
  platforms.

  With ELF, the alignment of a global variable in a shared library will
  get copied into an executables linked against it, if the executable even
  accesss the variable. So, it's not possible to implicitly increase
  alignment based on access patterns, or you'll break existing binaries.

  This happened to affect libc++'s std::cout symbol, for example. See
  thread: http://thread.gmane.org/gmane.comp.compilers.clang.devel/45311

  (This is a re-commit of r257719, without the bug reported in
  PR26144. I've tweaked the code to not assert-fail in
  enforceKnownAlignment when computeKnownBits doesn't recurse far enough
  to find the underlying Alloca/GlobalObject value.)

  Differential Revision: http://reviews.llvm.org/D16145

1 files changed, 20 insertions, 13 deletions

diff --git a/contrib/llvm/lib/Transforms/Utils/Local.cpp b/contrib/llvm/lib/Transforms/Utils/Local.cpp
index d2793e5..1cb65fc 100644
--- a/contrib/llvm/lib/Transforms/Utils/Local.cpp
+++ b/contrib/llvm/lib/Transforms/Utils/Local.cpp
@@ -944,37 +944,44 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
 static unsigned enforceKnownAlignment(Value *V, unsigned Align,
                                       unsigned PrefAlign,
                                       const DataLayout &DL) {
+  assert(PrefAlign > Align);
+
   V = V->stripPointerCasts();
 
   if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
+    // TODO: ideally, computeKnownBits ought to have used
+    // AllocaInst::getAlignment() in its computation already, making
+    // the below max redundant. But, as it turns out,
+    // stripPointerCasts recurses through infinite layers of bitcasts,
+    // while computeKnownBits is not allowed to traverse more than 6
+    // levels.
+    Align = std::max(AI->getAlignment(), Align);
+    if (PrefAlign <= Align)
+      return Align;
+
     // If the preferred alignment is greater than the natural stack alignment
     // then don't round up. This avoids dynamic stack realignment.
     if (DL.exceedsNaturalStackAlignment(PrefAlign))
       return Align;
-    // If there is a requested alignment and if this is an alloca, round up.
-    if (AI->getAlignment() >= PrefAlign)
-      return AI->getAlignment();
     AI->setAlignment(PrefAlign);
     return PrefAlign;
   }
 
   if (auto *GO = dyn_cast<GlobalObject>(V)) {
+    // TODO: as above, this shouldn't be necessary.
+    Align = std::max(GO->getAlignment(), Align);
+    if (PrefAlign <= Align)
+      return Align;
+
     // If there is a large requested alignment and we can, bump up the alignment
     // of the global.  If the memory we set aside for the global may not be the
     // memory used by the final program then it is impossible for us to reliably
     // enforce the preferred alignment.
-    if (!GO->isStrongDefinitionForLinker())
+    if (!GO->canIncreaseAlignment())
       return Align;
 
-    if (GO->getAlignment() >= PrefAlign)
-      return GO->getAlignment();
-    // We can only increase the alignment of the global if it has no alignment
-    // specified or if it is not assigned a section.  If it is assigned a
-    // section, the global could be densely packed with other objects in the
-    // section, increasing the alignment could cause padding issues.
-    if (!GO->hasSection() || GO->getAlignment() == 0)
-      GO->setAlignment(PrefAlign);
-    return GO->getAlignment();
+    GO->setAlignment(PrefAlign);
+    return PrefAlign;
   }
 
   return Align;