From: William Tambe <tambewilliam@gmail.com>
To: gcc-help <gcc-help@gcc.gnu.org>
Subject: Make GCC move instructions between a multi-cycle instruction and the next instruction that depends on its result.
Date: Tue, 22 Feb 2022 13:00:25 -0600 [thread overview]
Message-ID: <CAF8i9mOugP=Rovks8e4WSCJdbocDNt568S9rUEezrFW+kkxH0Q@mail.gmail.com> (raw)
In this CPU,
A multi-cycle instruction, once decoded, runs in parallel as other
decoded single/multi-cycle instructions.
A single/multi-cycle instruction takes two operands, where the first
operand receives the result
of computing both operands.
An example of multi-cycle instruction is "div".
An example of single-cycle instruction is "add".
GCC should be able to transform following:
````
add %0 %5
add %1 %6
div %4 %5 #<-- Multi-cycle instruction.
add %4 %7 #<-- Next instruction that depends on its result.
add %3 %7
add %2 %7
```
To:
```
div %4 %5 #<-- Multi-cycle instruction.
add %0 %5
add %1 %6
add %3 %7
add %2 %7
add %4 %7 #<-- Next instruction that depends on its result.
```
Without above transformation, `add %4 %7` would cause the cpu to wait
on `div %4 %5` when it could have executed instructions that do not
depend on the result of "div".
next reply other threads:[~2022-02-22 19:00 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-22 19:00 William Tambe [this message]
2022-02-22 21:15 ` William Tambe
2022-02-23 0:22 ` Segher Boessenkool
2022-02-23 0:51 ` William Tambe
2022-02-23 7:25 ` AW: " stefan
2022-02-23 15:07 ` William Tambe
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