Revert "[SelectionDAG] Do not second-guess alignment for alloca"

Breaks amdgpu buildbot https://lab.llvm.org/buildbot/#/builders/193
 23491

This reverts commit ffedf47d8b793e07317f82f9c2a5f5425ebb71ad.

1 files changed, 14 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
index 3a1a237..3e59d0d 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
@@ -128,7 +128,20 @@ void FunctionLoweringInfo::set(const Function &fn, MachineFunction &mf,
     for (const Instruction &I : BB) {
       if (const AllocaInst *AI = dyn_cast<AllocaInst>(&I)) {
         Type *Ty = AI->getAllocatedType();
-        Align Alignment = AI->getAlign();
+        Align TyPrefAlign = MF->getDataLayout().getPrefTypeAlign(Ty);
+        // The "specified" alignment is the alignment written on the alloca,
+        // or the preferred alignment of the type if none is specified.
+        //
+        // (Unspecified alignment on allocas will be going away soon.)
+        Align SpecifiedAlign = AI->getAlign();
+
+        // If the preferred alignment of the type is higher than the specified
+        // alignment of the alloca, promote the alignment, as long as it doesn't
+        // require realigning the stack.
+        //
+        // FIXME: Do we really want to second-guess the IR in isel?
+        Align Alignment =
+            std::max(std::min(TyPrefAlign, StackAlign), SpecifiedAlign);
 
         // Static allocas can be folded into the initial stack frame
         // adjustment. For targets that don't realign the stack, don't