V4 RegAlloc: change life-time intervals from closed to half-open.

There are two changes in this patch, that go hand-in-hand. First, when
re-numbering the statements in order of occurrence in the scheduled
basic-blocks, the (new) position is not stored in the statement itself,
but in the LifeTimeIntervals class. This makes it possible to re-use
information gathered during SSA formation or optimization.

The re-numbering itself has also changed, resulting in some minor
changes to the life-time interval calculation. The new numbering
is described in LifeTimeIntervals::renumber(). The reason is to make it
easy for the register allocator and stack-slot allocator to distinguish
between definition of a temporary and its uses. Example:

  20: %3 = %2 + %1
  22: print(%3)

If the life-time of %2 or %1 ends at 20, then at the point that %3 gets
assigned, it can re-use the storage occupied by %1 or %2. Also, when
both %1 and %2 need to get a register assigned (because they were
spilled to the stack, for example), %3 should be allocated "after" both
%1 and %2. So, instead of having a closed interval of [20-22] for %3, we
want to use an open interval of (20-22]. To simulate the "open" part, the
life-time of %3 is set to [21-22]. So, all statements live on even
positions, and temporaries defined by a statement start at
statmentPosition + 1.

Change-Id: I0eda2c653b0edf1a529bd0762d338b0ea9a66aa0
Sanity-Review: Qt Sanity Bot <qt_sanitybot@qt-project.org>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

1 files changed, 7 insertions, 2 deletions

diff --git a/src/qml/compiler/qv4jsir.cpp b/src/qml/compiler/qv4jsir.cpp
index 6b309cc282..9a1a8bab42 100644
--- a/src/qml/compiler/qv4jsir.cpp
+++ b/src/qml/compiler/qv4jsir.cpp
@@ -956,8 +956,7 @@ void IRPrinter::print(BasicBlock *bb)
         QTextStream os(&buf);
         QTextStream *prevOut = &os;
         std::swap(out, prevOut);
-        if (s->id() >= 0)
-            *out << s->id() << ": ";
+        addStmtNr(s);
         s->accept(this);
         if (s->location.isValid()) {
             out->flush();
@@ -1226,6 +1225,12 @@ QString IRPrinter::escape(const QString &s)
     return r;
 }
 
+void IRPrinter::addStmtNr(Stmt *s)
+{
+    if (s->id() >= 0)
+        *out << s->id() << ": ";
+}
+
 QString IRPrinter::dumpStart(const Expr *e)
 {
     if (e->type == UnknownType)