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comparison docs/AliasAnalysis.rst @ 148:63bd29f05246

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merged
author Shinji KONO <kono@ie.u-ryukyu.ac.jp>
date Wed, 14 Aug 2019 19:46:37 +0900
parents c2174574ed3a
children
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146:3fc4d5c3e21e 148:63bd29f05246
387 keep an AliasAnalysis consistent, however there's no way for a pass to declare 387 keep an AliasAnalysis consistent, however there's no way for a pass to declare
388 in its ``getAnalysisUsage`` that it does so. Some passes attempt to use 388 in its ``getAnalysisUsage`` that it does so. Some passes attempt to use
389 ``AU.addPreserved<AliasAnalysis>``, however this doesn't actually have any 389 ``AU.addPreserved<AliasAnalysis>``, however this doesn't actually have any
390 effect. 390 effect.
391 391
392 ``AliasAnalysisCounter`` (``-count-aa``) are implemented as ``ModulePass``
393 classes, so if your alias analysis uses ``FunctionPass``, it won't be able to
394 use these utilities. If you try to use them, the pass manager will silently
395 route alias analysis queries directly to ``BasicAliasAnalysis`` instead.
396
397 Similarly, the ``opt -p`` option introduces ``ModulePass`` passes between each 392 Similarly, the ``opt -p`` option introduces ``ModulePass`` passes between each
398 pass, which prevents the use of ``FunctionPass`` alias analysis passes. 393 pass, which prevents the use of ``FunctionPass`` alias analysis passes.
399 394
400 The ``AliasAnalysis`` API does have functions for notifying implementations when 395 The ``AliasAnalysis`` API does have functions for notifying implementations when
401 values are deleted or copied, however these aren't sufficient. There are many 396 values are deleted or copied, however these aren't sufficient. There are many
406 implementations can expect that they will be informed of any relevant ``Value`` 401 implementations can expect that they will be informed of any relevant ``Value``
407 before it appears in an alias query. However, popular clients such as ``GVN`` 402 before it appears in an alias query. However, popular clients such as ``GVN``
408 don't support this, and are known to trigger errors when run with the 403 don't support this, and are known to trigger errors when run with the
409 ``AliasAnalysisDebugger``. 404 ``AliasAnalysisDebugger``.
410 405
411 Due to several of the above limitations, the most obvious use for the
412 ``AliasAnalysisCounter`` utility, collecting stats on all alias queries in a
413 compilation, doesn't work, even if the ``AliasAnalysis`` implementations don't
414 use ``FunctionPass``. There's no way to set a default, much less a default
415 sequence, and there's no way to preserve it.
416
417 The ``AliasSetTracker`` class (which is used by ``LICM``) makes a 406 The ``AliasSetTracker`` class (which is used by ``LICM``) makes a
418 non-deterministic number of alias queries. This can cause stats collected by 407 non-deterministic number of alias queries. This can cause debugging techniques
419 ``AliasAnalysisCounter`` to have fluctuations among identical runs, for 408 involving pausing execution after a predetermined number of queries to be
420 example. Another consequence is that debugging techniques involving pausing 409 unreliable.
421 execution after a predetermined number of queries can be unreliable.
422 410
423 Many alias queries can be reformulated in terms of other alias queries. When 411 Many alias queries can be reformulated in terms of other alias queries. When
424 multiple ``AliasAnalysis`` queries are chained together, it would make sense to 412 multiple ``AliasAnalysis`` queries are chained together, it would make sense to
425 start those queries from the beginning of the chain, with care taken to avoid 413 start those queries from the beginning of the chain, with care taken to avoid
426 infinite looping, however currently an implementation which wants to do this can 414 infinite looping, however currently an implementation which wants to do this can
674 662
675 .. code-block:: bash 663 .. code-block:: bash
676 664
677 % opt -ds-aa -print-alias-sets -disable-output 665 % opt -ds-aa -print-alias-sets -disable-output
678 666
679 The ``-count-aa`` pass
680 ^^^^^^^^^^^^^^^^^^^^^^
681
682 The ``-count-aa`` pass is useful to see how many queries a particular pass is
683 making and what responses are returned by the alias analysis. As an example:
684
685 .. code-block:: bash
686
687 % opt -basicaa -count-aa -ds-aa -count-aa -licm
688
689 will print out how many queries (and what responses are returned) by the
690 ``-licm`` pass (of the ``-ds-aa`` pass) and how many queries are made of the
691 ``-basicaa`` pass by the ``-ds-aa`` pass. This can be useful when debugging a
692 transformation or an alias analysis implementation.
693
694 The ``-aa-eval`` pass 667 The ``-aa-eval`` pass
695 ^^^^^^^^^^^^^^^^^^^^^ 668 ^^^^^^^^^^^^^^^^^^^^^
696 669
697 The ``-aa-eval`` pass simply iterates through all pairs of pointers in a 670 The ``-aa-eval`` pass simply iterates through all pairs of pointers in a
698 function and asks an alias analysis whether or not the pointers alias. This 671 function and asks an alias analysis whether or not the pointers alias. This