* Role of unspec or unspec_volatile in a machine description pattern
@ 2019-09-11 10:15 William Tambe
2019-09-11 13:35 ` Ian Lance Taylor via gcc-help
0 siblings, 1 reply; 2+ messages in thread
From: William Tambe @ 2019-09-11 10:15 UTC (permalink / raw)
To: gcc-help
What is the role of unspec or unspec_volatile in a machine description pattern ?
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* Re: Role of unspec or unspec_volatile in a machine description pattern
2019-09-11 10:15 Role of unspec or unspec_volatile in a machine description pattern William Tambe
@ 2019-09-11 13:35 ` Ian Lance Taylor via gcc-help
0 siblings, 0 replies; 2+ messages in thread
From: Ian Lance Taylor via gcc-help @ 2019-09-11 13:35 UTC (permalink / raw)
To: William Tambe; +Cc: gcc-help
On Wed, Sep 11, 2019 at 3:15 AM William Tambe <tambewilliam@gmail.com> wrote:
>
> What is the role of unspec or unspec_volatile in a machine description pattern ?
They permit describing operations that otherwise cannot be described
in RTL, or for which there is no advantage to describing in RTL. For
example access to the TLS base pointer on x86, which is an offset from
a segment register. There is no reason to describe the segment
registers in RTL just to support this single instruction which always
looks the same.
Ian
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