[PATCH i386]: Enable push/pop in pro/epilogue for modern CPUs

[Xinliang David Li <davidxl@google.com>]

I noticed in prologue/epilogue, GCC prefers to use MOVs followed by a
SP adjustment instead of a sequence of pushes/pops. The preference to
the MOVs are good for old CPU micro-architectures (before pentium-4,
K10), because it breaks the data dependency.  In modern
micro-architecture, push/pop is implemented using a mechanism called
stack engine. The data dependency is removed by the hardware, and
push/pop becomes very cheap (1 uOp, 1 cycle latency), and they are
smaller. There is no longer the need to avoid using them.   This is
also what ICC does.

The following patch fixed the problem. It passes bootstrap/regression
test. OK to install?

thanks,

David

Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c (revision 194324)
+++ config/i386/i386.c (working copy)
@@ -1919,10 +1919,10 @@ static unsigned int initial_ix86_tune_fe
   m_P4_NOCONA | m_CORE2I7 | m_ATOM | m_AMD_MULTIPLE | m_GENERIC,

   /* X86_TUNE_PROLOGUE_USING_MOVE */
-  m_PPRO | m_CORE2I7 | m_ATOM | m_ATHLON_K8 | m_GENERIC,
+  m_PPRO | m_ATHLON_K8,

   /* X86_TUNE_EPILOGUE_USING_MOVE */
-  m_PPRO | m_CORE2I7 | m_ATOM | m_ATHLON_K8 | m_GENERIC,
+  m_PPRO | m_ATHLON_K8,

   /* X86_TUNE_SHIFT1 */
   ~m_486,


2012-12-08  Xinliang David Li  <davidxl@google.com>
           * config/i386/i386.c: Eanble push/pop in pro/epilogue for
moderen CPUs.