Reviewers: William Hesse,
Description:
Better range information for logical shift right >>>.
If the input range is positive and the shift count is constant
we can replace >>> with >> to compute the output range.
For negative inputs, we can only compute a range if the
result always fits into a signed int32.
BUG=v8:1510
Please review this at http://codereview.chromium.org/7489043/
SVN Base: http://v8.googlecode.com/svn/branches/bleeding_edge/
Affected files:
M src/hydrogen-instructions.h
M src/hydrogen-instructions.cc
Index: src/hydrogen-instructions.cc
===================================================================
--- src/hydrogen-instructions.cc (revision 8731)
+++ src/hydrogen-instructions.cc (working copy)
@@ -1230,6 +1230,30 @@
}
+Range* HShr::InferRange() {
+ if (right()->IsConstant()) {
+ HConstant* c = HConstant::cast(right());
+ if (c->HasInteger32Value()) {
+ int shift_count = c->Integer32Value() & 0x1f;
+ if (left()->range()->CanBeNegative()) {
+ // Only compute bounds if the result always fits into an int32.
+ return (shift_count >= 1)
+ ? new Range(0, static_cast<uint32_t>(0xffffffff) >>
shift_count)
+ : new Range();
+ } else {
+ // For positive inputs we can use the >> operator.
+ Range* result = (left()->range() != NULL)
+ ? left()->range()->Copy()
+ : new Range();
+ result->Sar(c->Integer32Value());
+ return result;
+ }
+ }
+ }
+ return HValue::InferRange();
+}
+
+
Range* HShl::InferRange() {
if (right()->IsConstant()) {
HConstant* c = HConstant::cast(right());
Index: src/hydrogen-instructions.h
===================================================================
--- src/hydrogen-instructions.h (revision 8731)
+++ src/hydrogen-instructions.h (working copy)
@@ -252,6 +252,7 @@
void AddConstant(int32_t value);
void Sar(int32_t value);
void Shl(int32_t value);
+ void Shr(int32_t value);
bool AddAndCheckOverflow(Range* other);
bool SubAndCheckOverflow(Range* other);
bool MulAndCheckOverflow(Range* other);
@@ -3060,6 +3061,7 @@
HShr(HValue* context, HValue* left, HValue* right)
: HBitwiseBinaryOperation(context, left, right) { }
+ virtual Range* InferRange();
virtual HType CalculateInferredType();
DECLARE_CONCRETE_INSTRUCTION(Shr)
--
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