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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@141599 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 6 insertions, 0 deletions

diff --git a/include/llvm/Target/TargetData.h b/include/llvm/Target/TargetData.h
index 315bee96ba..e34094ddb7 100644
--- a/include/llvm/Target/TargetData.h
+++ b/include/llvm/Target/TargetData.h
@@ -70,6 +70,7 @@ private:
   unsigned      PointerMemSize;        ///< Pointer size in bytes
   unsigned      PointerABIAlign;       ///< Pointer ABI alignment
   unsigned      PointerPrefAlign;      ///< Pointer preferred alignment
+  unsigned      StackNaturalAlign;     ///< Stack natural alignment
 
   SmallVector<unsigned char, 8> LegalIntWidths; ///< Legal Integers.
   
@@ -163,6 +164,11 @@ public:
     return !isLegalInteger(Width);
   }
 
+  /// Returns true if the given alignment exceeds the natural stack alignment.
+  bool exceedsNaturalStackAlignment(unsigned Align) const {
+    return (StackNaturalAlign != 0) && (Align > StackNaturalAlign);
+  }
+
   /// fitsInLegalInteger - This function returns true if the specified type fits
   /// in a native integer type supported by the CPU.  For example, if the CPU
   /// only supports i32 as a native integer type, then i27 fits in a legal