[SelectionDAG] Do not second-guess alignment for alloca

Alignment of an alloca in IR can be lower than the preferred alignment
on purpose, but this override essentially treats the preferred
alignment as the minimum alignment.

The patch changes this behavior to always use the specified
alignment. If alignment is not set explicitly in LLVM IR, it is set to
DL.getPrefTypeAlign(Ty) in computeAllocaDefaultAlign.

Tests are changed as well: explicit alignment is increased to match
the preferred alignment if it changes output, or omitted when it is
hard to determine the right value (e.g. for pointers, some structs, or
weird types).

Differential Revision: https://reviews.llvm.org/D135462

1 files changed, 1 insertions, 14 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
index 3e59d0d..3a1a237 100644
--- a/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp
@@ -128,20 +128,7 @@ 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 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);
+        Align Alignment = AI->getAlign();
 
         // Static allocas can be folded into the initial stack frame
         // adjustment. For targets that don't realign the stack, don't