GNU Compiler Collection (GCC) Internals: Define Subst Output Template

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#### 16.21.3 Generation of output template in `define_subst`

If all necessary checks for `define_subst` application pass, a new RTL-pattern, based on the output-template, is created to replace the old template. Like in input-patterns, meanings of some RTL expressions are changed when they are used in output-patterns of a `define_subst`. Thus, `match_dup` is used for copying the whole expression from the original pattern, which matched corresponding `match_operand` from the input pattern.

`match_dup N` is used in the output template to be replaced with the expression from the original pattern, which matched `match_operand N` from the input pattern. As a consequence, `match_dup` cannot be used to point to `match_operand`s from the output pattern, it should always refer to a `match_operand` from the input pattern.

In the output template one can refer to the expressions from the original pattern and create new ones. For instance, some operands could be added by means of standard `match_operand`.

After replacing `match_dup` with some RTL-subtree from the original pattern, it could happen that several `match_operand`s in the output pattern have the same indexes. It is unknown, how many and what indexes would be used in the expression which would replace `match_dup`, so such conflicts in indexes are inevitable. To overcome this issue, `match_operands` and `match_operators`, which were introduced into the output pattern, are renumerated when all `match_dup`s are replaced.

Number of alternatives in `match_operand`s introduced into the output template `M` could differ from the number of alternatives in the original pattern `N`, so in the resultant pattern there would be `N*M` alternatives. Thus, constraints from the original pattern would be duplicated `N` times, constraints from the output pattern would be duplicated `M` times, producing all possible combinations.

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