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### 15.6 IV analysis on RTL

The induction variable on RTL is simple and only allows analysis of affine induction variables, and only in one loop at once. The interface is declared in `cfgloop.h`. Before analyzing induction variables in a loop L, `iv_analysis_loop_init` function must be called on L. After the analysis (possibly calling `iv_analysis_loop_init` for several loops) is finished, `iv_analysis_done` should be called. The following functions can be used to access the results of the analysis:

- `iv_analyze`: Analyzes a single register used in the given insn. If no use of the register in this insn is found, the following insns are scanned, so that this function can be called on the insn returned by get_condition.
- `iv_analyze_result`: Analyzes result of the assignment in the given insn.
- `iv_analyze_expr`: Analyzes a more complicated expression. All its operands are analyzed by `iv_analyze`, and hence they must be used in the specified insn or one of the following insns.

The description of the induction variable is provided in `struct rtx_iv`. In order to handle subregs, the representation is a bit complicated; if the value of the `extend` field is not `UNKNOWN`, the value of the induction variable in the i-th iteration is

         delta + mult * extend_{extend_mode} (subreg_{mode} (base + i * step)),

with the following exception: if `first_special` is true, then the value in the first iteration (when `i` is zero) is `delta + mult * base`. However, if `extend` is equal to `UNKNOWN`, then `first_special` must be false, `delta` 0, `mult` 1 and the value in the i-th iteration is

         subreg_{mode} (base + i * step)

The function `get_iv_value` can be used to perform these calculations.