define_subst Example ¶To illustrate how define_subst works, let us examine a simple
template transformation.
Suppose there are two kinds of instructions: one that touches flags and
the other that does not. The instructions of the second type could be
generated with the following define_subst:
(define_subst "add_clobber_subst"
[(set (match_operand:SI 0 "" "")
(match_operand:SI 1 "" ""))]
""
[(set (match_dup 0)
(match_dup 1))
(clobber (reg:CC FLAGS_REG))])
This define_subst can be applied to any RTL pattern containing
set of mode SI and generates a copy with clobber when it is
applied.
Assume there is an RTL template for a max instruction to be used
in define_subst mentioned above:
(define_insn "maxsi"
[(set (match_operand:SI 0 "register_operand" "=r")
(max:SI
(match_operand:SI 1 "register_operand" "r")
(match_operand:SI 2 "register_operand" "r")))]
""
"max\t{%2, %1, %0|%0, %1, %2}"
[…])
To mark the RTL template for define_subst application,
subst-attributes are used. They should be declared in advance:
(define_subst_attr "add_clobber_name" "add_clobber_subst" "_noclobber" "_clobber")
Here ‘add_clobber_name’ is the attribute name,
‘add_clobber_subst’ is the name of the corresponding
define_subst, the third argument (‘_noclobber’) is the
attribute value that would be substituted into the unchanged version of
the source RTL template, and the last argument (‘_clobber’) is the
value that would be substituted into the second, transformed,
version of the RTL template.
Once the subst-attribute has been defined, it should be used in RTL
templates which need to be processed by the define_subst. So,
the original RTL template should be changed:
(define_insn "maxsi<add_clobber_name>"
[(set (match_operand:SI 0 "register_operand" "=r")
(max:SI
(match_operand:SI 1 "register_operand" "r")
(match_operand:SI 2 "register_operand" "r")))]
""
"max\t{%2, %1, %0|%0, %1, %2}"
[…])
The result of the define_subst usage would look like the following:
(define_insn "maxsi_noclobber"
[(set (match_operand:SI 0 "register_operand" "=r")
(max:SI
(match_operand:SI 1 "register_operand" "r")
(match_operand:SI 2 "register_operand" "r")))]
""
"max\t{%2, %1, %0|%0, %1, %2}"
[…])
(define_insn "maxsi_clobber"
[(set (match_operand:SI 0 "register_operand" "=r")
(max:SI
(match_operand:SI 1 "register_operand" "r")
(match_operand:SI 2 "register_operand" "r")))
(clobber (reg:CC FLAGS_REG))]
""
"max\t{%2, %1, %0|%0, %1, %2}"
[…])