RFR: 8273563: Improve performance of implicit exceptions with -XX:-OmitStackTraceInFastThrow

[Martin Doerr]

On Mon, 13 Sep 2021 10:05:16 GMT, Volker Simonis <simonis at openjdk.org> wrote:

> Currently, if running with `-XX:-OmitStackTraceInFastThrow`, C2 has no possibility to create implicit exceptions like AIOOBE, NullPointerExceptions, etc. in compiled code. This means that such methods will always be deoptimized and re-executed in the interpreter if such exceptions are happening.
> 
> If implicit exceptions are used for normal control flow, that can have a dramatic impact on performance. A prominent example for such code is [Tomcat's `HttpParser::isAlpha()` method](https://github.com/apache/tomcat/blob/26ba86cdbd40ca718e43b82e62b3eb49d004c3d6/java/org/apache/tomcat/util/http/parser/HttpParser.java#L266-L274):
> 
>     public static boolean isAlpha(int c) {
>         try {
>             return IS_ALPHA[c];
>         } catch (ArrayIndexOutOfBoundsException ex) {
>             return false;
>         }
>     }
> 
> 
> ### Solution
> 
> Instead of deoptimizing and resorting to the interpreter, we can generate code which allocates and initializes the corresponding exceptions right in compiled code. This results in a ten-times performance improvement for the above code:
> 
> -XX:-OmitStackTraceInFastThrow -XX:-OptimizeImplicitExceptions
> Benchmark                 (exceptionProbability)  Mode  Cnt      Score      Error  Units
> ImplicitExceptions.bench                     0.0  avgt    5      1.430 ±    0.353  ns/op
> ImplicitExceptions.bench                    0.33  avgt    5   3563.038 ±   77.358  ns/op
> ImplicitExceptions.bench                    0.66  avgt    5   8609.693 ± 1205.104  ns/op
> ImplicitExceptions.bench                    1.00  avgt    5  12842.401 ± 1022.728  ns/op
> 
> -XX:-OmitStackTraceInFastThrow -XX:+OptimizeImplicitExceptions
> Benchmark                 (exceptionProbability)  Mode  Cnt      Score      Error  Units
> ImplicitExceptions.bench                     0.0  avgt    5     1.432  ±    0.352  ns/op
> ImplicitExceptions.bench                    0.33  avgt    5   355.723  ±   16.641  ns/op
> ImplicitExceptions.bench                    0.66  avgt    5   887.068  ±  166.728  ns/op
> ImplicitExceptions.bench                    1.00  avgt    5  1274.418  ±   88.235  ns/op
> 
> 
> ### Implementation details
> 
> - The new optimization is guarded by the option `OptimizeImplicitExceptions` which is on by default.
> - In `GraphKit::builtin_throw()` we can't simply use `CallGenerator::for_direct_call()` to create a `DirectCallGenerator` for the call to the exception's `<init>` function because `DirectCallGenerator` assumes in various places that calls are only issued at `invoke*` bytecodes. This is is not true in genral for bytecode which can cause an implicit exception. 
> - Instead, we manually wire up the call based on the code in `DirectCallGenerator::generate()`.
> - We use a similar trick like for method handle intrinsics where the callee from the bytecode is replaced by a direct call and this fact is recorded in the call's `_override_symbolic_info` field. For calling constructors of implicit exceptions I've introduced the new field `_implicit_exception_init`. This field is also used in various assertions to prevent queries for the bytecode's symbolic method information which doesn't exist because we're not at an `invoke*` bytecode at the place where we generate the call.
> - The PR contains a micro-benchmark which compares the old and the new implementation for [Tomcat's `HttpParser::isAlpha()` method](https://github.com/apache/tomcat/blob/26ba86cdbd40ca718e43b82e62b3eb49d004c3d6/java/org/apache/tomcat/util/http/parser/HttpParser.java#L266-L274). Except for the trivial case where the exception probability is 0 (i.e. no exceptions are happening at all) the new implementation is about 10 times faster.

This looks like a great idea. I have a few minor remarks / suggestions.

src/hotspot/share/ci/ciEnv.cpp line 375:

> 373: // ------------------------------------------------------------------
> 374: // helper for -XX:+OptimizeImplicitExceptions
> 375: ciInstanceKlass* ciEnv::exception_instanceKlass_for_reason(Deoptimization::DeoptReason reason, bool aastore) {

Better `is_aastore` or pass Bytecode?

src/hotspot/share/opto/graphKit.cpp line 631:

> 629:       Node* ex_node = new_instance(makecon(ex_type), NULL, NULL, true);
> 630:       set_argument(0, ex_node);
> 631:       ciMethod* init =  ex_ciInstKlass->find_method(ciSymbol::make("<init>"), ciSymbol::make("()V"));

Extra whitespace.

src/hotspot/share/runtime/globals.hpp line 645:

> 643:           "Omit backtraces for some 'hot' exceptions in optimized code")    \
> 644:                                                                             \
> 645:   product(bool, OptimizeImplicitExceptions, true,                           \

Should it be a diagnostic flag? Regular product flags require a CSR.

src/hotspot/share/runtime/sharedRuntime.cpp line 1096:

> 1094:     bc = bytecode.invoke_code();
> 1095:   }
> 1096:   else {

Coding style: newline before `else`

-------------

PR: https://git.openjdk.java.net/jdk/pull/5488