Add getJitMethodEntryAlignmentThreshold API

Determines the byte threshold at which the JIT-to-JIT method entry
point boundary alignment will not be performed. If the JIT-to-JIT
method entry point is already close to the boundary then it may not
make sense to perform the boundary alignment as much code cache can be
wasted. This threshold can be used to avoid such situations.

Signed-off-by: Filip Jeremic <fjeremic@ca.ibm.com>

compiler/codegen/OMRCodeGenerator.cpp

@@ -2160,17 +2160,23 @@ OMR::CodeGenerator::alignBinaryBufferCursor()
    {
    uintptr_t boundary = self()->getJitMethodEntryAlignmentBoundary();
 
-   /* Align cursor to boundary */
+   // Align cursor to boundary as long as it meets the threshold
    if (boundary && (boundary & boundary - 1) == 0)
       {
       uintptr_t round = boundary - 1;
       uintptr_t offset = self()->getPreJitMethodEntrySize();
 
-      _binaryBufferCursor += offset;
-      _binaryBufferCursor = (uint8_t *)(((uintptr_t)_binaryBufferCursor + round) & ~round);
-      _binaryBufferCursor -= offset;
-      self()->setJitMethodEntryPaddingSize(_binaryBufferCursor - _binaryBufferStart);
-      memset(_binaryBufferStart, 0, self()->getJitMethodEntryPaddingSize());
+      uint8_t* alignedBufferCursor = _binaryBufferCursor;
+      alignedBufferCursor += offset;
+      alignedBufferCursor = (uint8_t *)(((uintptr_t)alignedBufferCursor + round) & ~round);
+      alignedBufferCursor -= offset;
+
+      if (alignedBufferCursor - _binaryBufferCursor <= self()->getJitMethodEntryAlignmentThreshold())
+         {
+         _binaryBufferCursor = alignedBufferCursor;
+         self()->setJitMethodEntryPaddingSize(_binaryBufferCursor - _binaryBufferStart);
+         memset(_binaryBufferStart, 0, self()->getJitMethodEntryPaddingSize());
+         }
       }
 
    return _binaryBufferCursor;
@@ -2182,6 +2188,12 @@ OMR::CodeGenerator::getJitMethodEntryAlignmentBoundary()
    return 1;
    }
 
+uint32_t
+OMR::CodeGenerator::getJitMethodEntryAlignmentThreshold()
+   {
+   return self()->getJitMethodEntryAlignmentBoundary();
+   }
+
 int32_t
 OMR::CodeGenerator::setEstimatedLocationsForSnippetLabels(int32_t estimatedSnippetStart)
    {

compiler/codegen/OMRCodeGenerator.hpp

@@ -766,6 +766,17 @@ class OMR_EXTENSIBLE CodeGenerator
     *     specified to be \c x and the JIT-to-JIT method entry point to be \c y then <c>y & (x - 1) == 0</c>.
     */
    uint32_t getJitMethodEntryAlignmentBoundary();
+
+   /** \brief
+    *     Determines the byte threshold at which the JIT-to-JIT method entry point boundary alignment will not be
+    *     performed. If the JIT-to-JIT method entry point is already close to the boundary then it may not make sense
+    *     to perform the boundary alignment as much code cache can be wasted. This threshold can be used to avoid such
+    *     situations.
+    *
+    *  \note
+    *     This value must be less than or equal to the boundary returned via \see getJitMethodEntryAlignmentBoundary.
+    */
+   uint32_t getJitMethodEntryAlignmentThreshold();
 
    uint32_t getJitMethodEntryPaddingSize() {return _jitMethodEntryPaddingSize;}
    uint32_t setJitMethodEntryPaddingSize(uint32_t s) {return (_jitMethodEntryPaddingSize = s);}

compiler/z/codegen/OMRCodeGenerator.cpp

@@ -4699,6 +4699,12 @@ OMR::Z::CodeGenerator::getJitMethodEntryAlignmentBoundary()
    return 256;
    }
 
+uint32_t
+OMR::Z::CodeGenerator::getJitMethodEntryAlignmentThreshold()
+   {
+   return 192;
+   }
+
 /**
  * This function sign extended the specified number of high order bits in the register.
  */

compiler/z/codegen/OMRCodeGenerator.hpp

@@ -783,6 +783,8 @@ class OMR_EXTENSIBLE CodeGenerator : public OMR::CodeGenerator
 
    uint32_t getJitMethodEntryAlignmentBoundary();
 
+   uint32_t getJitMethodEntryAlignmentThreshold();
+
    // LL: move to .cpp
    bool arithmeticNeedsLiteralFromPool(TR::Node *node);