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Re: question about speculative scheduling in gcc

From: Maxim Kuvyrkov <maxim at codesourcery dot com>
To: "Amker.Cheng" <amker dot cheng at gmail dot com>
Cc: gcc at gcc dot gnu dot org
Date: Sun, 20 Sep 2009 11:43:40 +0400
Subject: Re: question about speculative scheduling in gcc
References: <e8b251c80909192109q5ecb2cd5i2f682b847fb51bd1@mail.gmail.com>

Amker.Cheng wrote:

Hi :
I'm puzzled when looking into speculative scheduling in gcc, the 4.2.4 version.

First, I noticed the document describing IBM haifa instruction
scheduler(as PowerPC Reference Compiler Optimization Project).

It presents that the instruction motion from bb s(dominated by t)
to t is speculative when split_blocks(s, t) not empty.

Second, There is SCED_FLAGS like DO_SPECULATION in codes.

These are two different types of speculative optimizations.


Here goes questions.
1, Does the DO_SPECULATION flag constrol whether do the
    mentioned speculative motion or not?

DO_SPECULATION flag controls generation of IA64 data and control speculative instructions. It is not used on other architectures.

Speculative instruction moves from the split blocks are controlled by flag_schedule_speculative.

--
Maxim

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