Poor man's fast breakpoints for a 2.3x debugger speedup.
<https://webkit.org/b/122836>
Reviewed by Geoffrey Garen.
Previously we gained back some performance (run at baseline JIT speeds)
when the WebInspector is opened provided no breakpoints are set. This
was achieved by simply skipping all op_debug callbacks to the debugger
if no breakpoints are set. If any breakpoints are set, the debugger will
set a m_needsOpDebugCallbacks flag which causes the callbacks to be
called, and we don't get the baseline JIT speeds anymore.
With this patch, we will now track the number of breakpoints set in the
CodeBlock that they are set in. The LLINT and baseline JIT code will
check CodeBlock::m_numBreakpoints to determine if the op_debug callbacks
need to be called. With this, we will only enable op_debug callbacks for
CodeBlocks that need it i.e. those with breakpoints set in them.
Debugger::m_needsOpDebugCallbacks is now obsoleted. The LLINT and baseline
JIT code still needs to check Debugger::m_shouldPause to determine if the
debugger is in stepping mode and hence, needs op_debug callbacks enabled
for everything until the debugger "continues" the run and exit stepping
mode.
Also in this patch, I fixed a regression in DOM breakpoints which relies
Debugger::breakProgram() to pause the debugger.
(JSC::CodeBlock::dumpBytecode):
- Missed accounting for op_debug's new hasBreakpointFlag operand here when
it was added.
(JSC::CodeBlock::CodeBlock):
(JSC::CodeBlock::hasOpDebugForLineAndColumn):
- This is needed in Debugger::toggleBreakpoint() to determine if a
breakpoint falls within a CodeBlock or not. Simply checking the bounds
of the CodeBlock is insufficient. For example, let's say we have the
following JS code:
begin global scope
function f1() {
function f2() {
... set breakpoint here.
}
}
end global scope
Using the CodeBlock bounds alone, the breakpoint above will to appear
to be in the global program CodeBlock, and the CodeBlocks for function
f1() and f2(). With CodeBlock::hasOpDebugForLineAndColumn() we can
rule out the global program CodeBlock and f1(), and only apply the
breakpoint to f2(0 where it belongs.
CodeBlock::hasOpDebugForLineAndColumn() works by iterating over all
the opcodes in the CodeBlock to look for op_debug's. For each op_debug,
it calls CodeBlock::expressionRangeForBytecodeOffset() to do a binary
seach to get the line and column info for that op_debug. This is a
N * log(N) algorithm. However, a quick hands on test using the
WebInspector (with this patch applied) to exercise setting, breaking
on, and clearing breakpoints, as well as stepping through some code
shows no noticeable degradation of the user experience compared to the
baseline without this patch.
(JSC::CodeBlock::numBreakpoints):
(JSC::CodeBlock::numBreakpointsOffset):
(JSC::CodeBlock::addBreakpoint):
(JSC::CodeBlock::removeBreakpoint):
(JSC::CodeBlock::clearAllBreakpoints):
- debugger/Breakpoint.h:
- defined Breakpoint::unspecifiedColumn so that we can explicitly indicate
when the WebInspector was setting a line breakpoint and did not provide
a column value. CodeBlock::hasOpDebugForLineAndColumn() needs this
information in order to loosen its matching criteria for op_debug
bytecodes for the specified breakpoint line and column values provided
by the debugger.
Previously, we just hijack a 0 value column as an unspecified column.
However, the WebInspector operates on 0-based ints for column values.
Hence, 0 should be a valid column value and should not be hijacked to
mean an unspecified column.
(JSC::Debugger::Debugger):
- added tracking of the VM that the debugger is used with. This is
needed by Debugger::breakProgram().
The VM pointer is attained from the first JSGlobalObject that the debugger
attaches to. When the debugger detaches from the last JSGlobalObject, it
will nullify its VM pointer to allow a new one to be set on the next
attach.
We were always only using each debugger instance with one VM. This change
makes it explicit with an assert to ensure that all globalObjects that
the debugger attaches to beongs to the same VM.
(JSC::Debugger::attach):
(JSC::Debugger::detach):
(JSC::Debugger::setShouldPause):
(JSC::Debugger::registerCodeBlock):
(JSC::Debugger::unregisterCodeBlock):
- registerCodeBlock() is responsible for applying pre-existing breakpoints
to new CodeBlocks being installed. Similarly, unregisterCodeBlock()
clears the breakpoints.
(JSC::Debugger::toggleBreakpoint):
- This is the workhorse function that checks if a breakpoint falls within
a CodeBlock or not. If it does, then it can either enable or disable
said breakpoint in the CodeBlock. In the current implementation,
enabling/disabling the breakpoint simply means incrementing/decrementing
the CodeBlock's m_numBreakpoints.
(JSC::Debugger::applyBreakpoints):
(JSC::Debugger::ToggleBreakpointFunctor::ToggleBreakpointFunctor):
(JSC::Debugger::ToggleBreakpointFunctor::operator()):
(JSC::Debugger::toggleBreakpoint):
- Iterates all relevant CodeBlocks and apply the specified breakpoint
if appropriate. This is called when a new breakpoint is being defined
by the WebInspector and needs to be applied to an already installed
CodeBlock.
(JSC::Debugger::setBreakpoint):
(JSC::Debugger::removeBreakpoint):
(JSC::Debugger::hasBreakpoint):
(JSC::Debugger::ClearBreakpointsFunctor::ClearBreakpointsFunctor):
(JSC::Debugger::ClearBreakpointsFunctor::operator()):
(JSC::Debugger::clearBreakpoints):
(JSC::Debugger::breakProgram):
- Fixed a regression that broke DOM breakpoints. The issue is that with
the skipping of op_debug callbacks, we don't always have an updated
m_currentCallFrame. Normally, m_currentCallFrame is provided as arg
in the op_debug callback. In this case, we can get the CallFrame* from
m_vm->topCallFrame.
(JSC::Debugger::updateCallFrameAndPauseIfNeeded):
(JSC::Debugger::pauseIfNeeded):
(JSC::Debugger::willExecuteProgram):
(JSC::Debugger::Debugger):
(JSC::Debugger::shouldPause):
(JSC::CodeBlockSet::iterate):
(JSC::Heap::forEachCodeBlock):
- Added utility to iterate all CodeBlocks in the heap / VM.
- interpreter/Interpreter.cpp:
(JSC::Interpreter::debug):
(JSC::JIT::emit_op_debug):
(JSC::JIT::emit_op_debug):
- llint/LowLevelInterpreter.asm:
- These now checks CodeBlock::m_numBreakpoints and Debugger::m_shouldPause
instead of Debugger::m_needsOpDebugCallbacks.
(JSC::ScriptExecutable::installCode):
