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Re: [JAVA PATCH] Enable more array bounds check elimination

On 02/22/2016 11:10 AM, wrote:

It has been a while since my last contribution.  The following patch allows GCC's optimizers
to more aggressively eliminate and optimize java array bounds checks.  The results are
quite impressive, for example producing a 26% performance improvement on the
benchmark given at on x86_64-pc-linux-gnu, reducing the
runtime for 1 million iterations from 31.5 seconds on trunk, to 25.0s with this patch, in fact
eliminating all array bounds checks.  This is close to the 22.3s of an equivalent C/C++
implementation, and significantly closes the gap to Java HotSpot(TM) JIT at 23.0 seconds.

The approach is to provide sufficient information in the gimple generated by the gcj front-end
to allow the optimizers to do their thing.  For array allocations of constant length, I propose
generating an additional (cheap) write to the array length field returned from _Jv_NewPrimArray,
which is then sufficient to allow this value to propagate throughout the optimizers.

This is probably best explained by a simple example.  Consider the array initializer below:
Thanks.  The example helped a lot.

At a very high level, you should be aware of a general belief that GCJ's life is limited. There's been various calls to deprecate it. So spending a lot of time optimizing GCJ's output may not be the best use of your skills :-)

With the patch below, we now generate the much more informative .004t.gimple for this:

    D.926 = _Jv_NewPrimArray (&_Jv_intClass, 3);
    D.926->length = 3;
Essentially you're just storing back into the result the length that we'd passed to the allocator. Cute. Good to see that all the work we've done to propagate the RHS of that kind of statement into uses has paid off.

Presumably there's no reasonable way this could fail (like you're getting objects from a readonly part of memory), or the result gets used in some other thread which changes its size prior to the re-storing of the initial size?

Achieving this result required two minor tweaks.   The first is to allow the array length constant
to reach the newarray call, by allowing constants to remain on the quickstack.  This allows the
call to _Jv_NewPrimArray to have a constant integer argument instead of the opaque #slot#0#0.
Then in the code that constructs the call to _Jv_NewPrimArray we wrap it in a COMPOUND_EXPR
allowing us to insert the superfluous, but helpful, write to the length field.

Whilst working on this improvement I also noticed that the array bounds checks we were
initially generating could also be improved.  Currently, an array bound check in 004t.gimple
looks like:

    D.925 = MEM[(struct int[] *)_ref_1_4.6].length;
    D.926 = (unsigned int) D.925;
    if (_slot_2_5.9 >= D.926) goto <D.927>; else goto <D.921>;
    _Jv_ThrowBadArrayIndex (_slot_2_5.8);
    if (0 != 0) goto <D.928>; else goto <D.921>;
    iftmp.7 = 1;
    goto <D.922>;
    iftmp.7 = 0;

Notice the unnecessary "0 != 0" and the dead assignments to iftmp.7 (which is unused).
FWIW, we're generally moving away from optimization in the language front-ends -- in particular folding, which you introduce in this patch on the array index. Given the trajectory of GCJ I'm not going to worry about it though.

With the patch below, we now not only avoid this conditional but also use __builtin_expect
to inform the compiler that throwing an BadArrayIndex exception is typically unlikely.  i.e.
Sounds like a good thing as well.

    D.930 = MEM[(struct int[] *)_ref_1_4.4].length;
    D.931 = D.930 <= 1;
    D.932 = __builtin_expect (D.931, 0);
    if (D.932 != 0) goto <D.933>; else goto <D.934>;
    _Jv_ThrowBadArrayIndex (0);

The following patch has been tested on x86_64-pc-linux-gnu with a full make bootstrap
and make check, with no new failures/regressions.

Please let me know what you think (for stage 1 once it reopens)?

Roger Sayle, Ph.D.
CEO and founder
NextMove Software Limited
Registered in England No. 07588305
Registered Office: Innovation Centre (Unit 23), Cambridge Science Park, Cambridge CB4 0EY

2016-02-21  Roger Sayle  <>

	* expr.c (push_value): Only call flush_quick_stack for non-constant
	(build_java_throw_out_of_bounds_exception): No longer wrap calls
	to _Jv_ThowBadArrayIndex in a COMPOUND_EXPR as no longer needed.
	(java_check_reference): Annotate COND_EXPR with __builtin_expect
	to indicate that calling _Jv_ThrowNullPointerException is unlikely.
	(build_java_arrayaccess): Construct an unlikely COND_EXPR instead
	of a TRUTH_ANDIF_EXPR in a COMPOUND_EXPR.  Only generate array
	index MULT_EXPR when size_exp is not unity.
	(build_array_length_annotation): When optimizing, generate a write
	to the allocated array's length field to expose constant lengths
	to GCC's optimizers.
	(build_newarray): Call new build_array_length_annotation.
	(build_anewarray): Likewise.
Looks generally OK. There's a whitespace nit in the call to build3 in build_java_arrayaccess (missing space between the function name and open paren).

I think this is OK for trunk after fixing the whitespace nit.


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