Add a natural stack alignment field to TargetData, and prevent InstCombine from

promoting allocas to preferred alignments that exceed the natural
alignment. This avoids some potentially expensive dynamic stack realignments.

The natural stack alignment is set in target data strings via the "S<size>"
option. Size is in bits and must be a multiple of 8. The natural stack alignment
defaults to "unspecified" (represented by a zero value), and the "unspecified"
value does not prevent any alignment promotions. Target maintainers that care
about avoiding promotions should explicitly add the "S<size>" option to their
target data strings.

llvm-svn: 141599

1 files changed, 6 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index be0aa825001..7034feb227a 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -721,10 +721,14 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
 /// their preferred alignment from the beginning.
 ///
 static unsigned enforceKnownAlignment(Value *V, unsigned Align,
-                                      unsigned PrefAlign) {
+                                      unsigned PrefAlign, const TargetData *TD) {
   V = V->stripPointerCasts();
 
   if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
+    // If the preferred alignment is greater than the natural stack alignment
+    // then don't round up. This avoids dynamic stack realignment.
+    if (TD && TD->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();
@@ -775,7 +779,7 @@ unsigned llvm::getOrEnforceKnownAlignment(Value *V, unsigned PrefAlign,
   Align = std::min(Align, +Value::MaximumAlignment);
   
   if (PrefAlign > Align)
-    Align = enforceKnownAlignment(V, Align, PrefAlign);
+    Align = enforceKnownAlignment(V, Align, PrefAlign, TD);
     
   // We don't need to make any adjustment.
   return Align;