### 8.5 RTL passes

The following briefly describes the rtl generation and optimization passes that are run after tree optimization.

- RTL generation

  The source files for RTL generation include `stmt.c`, `calls.c`, `expr.c`, `explow.c`, `expmed.c`, `function.c`, `optabs.c` and `emit-rtl.c`. Also, the file `insn-emit.c`, generated from the machine description by the program `genemit`, is used in this pass. The header file `expr.h` is used for communication within this pass.

  The header files `insn-flags.h` and `insn-codes.h`, generated from the machine description by the programs `genflags` and `gencodes`, tell this pass which standard names are available for use and which patterns correspond to them.

- Generate exception handling landing pads

  This pass generates the glue that handles communication between the exception handling library routines and the exception handlers within the function. Entry points in the function that are invoked by the exception handling library are called landing pads. The code for this pass is located within `except.c`.

- Cleanup control flow graph

  This pass removes unreachable code, simplifies jumps to next, jumps to jump, jumps across jumps, etc. The pass is run multiple times. For historical reasons, it is occasionally referred to as the “jump optimization pass”. The bulk of the code for this pass is in `cfgcleanup.c`, and there are support routines in `cfgrtl.c` and `jump.c`.

- Common subexpression elimination

  This pass removes redundant computation within basic blocks, and optimizes addressing modes based on cost. The pass is run twice. The source is located in `cse.c`.

- Global common subexpression elimination.

  This pass performs two different types of GCSE depending on whether you are optimizing for size or not (LCM based GCSE tends to increase code size for a gain in speed, while Morel-Renvoise based GCSE does not). When optimizing for size, GCSE is done using Morel-Renvoise Partial Redundancy Elimination, with the exception that it does not try to move invariants out of loops—that is left to the loop optimization pass. If MR PRE GCSE is done, code hoisting (aka unification) is also done, as well as load motion. If you are optimizing for speed, LCM (lazy code motion) based GCSE is done. LCM is based on the work of Knoop, Ruthing, and Steffen. LCM based GCSE also does loop invariant code motion. We also perform load and store motion when optimizing for speed. Regardless of which type of GCSE is used, the GCSE pass also performs global constant and copy propagation. The source file for this pass is `gcse.c`, and the LCM routines are in `lcm.c`.

- Loop optimization

  This pass performs several loop related optimizations. The source files `cfgloopanal.c` and `cfgloopmanip.c` contain generic loop analysis and manipulation code. Initialization and finalization of loop structures is handled by `loop-init.c`. A loop invariant motion pass is implemented in `loop-invariant.c`. Basic block level optimizations—unrolling, peeling and unswitching loops— are implemented in `loop-unswitch.c` and `loop-unroll.c`. Replacing of the exit condition of loops by special machine-dependent instructions is handled by `loop-doloop.c`.

- Jump bypassing

  This pass is an aggressive form of GCSE that transforms the control flow graph of a function by propagating constants into conditional branch instructions. The source file for this pass is `gcse.c`.

- If conversion

  This pass attempts to replace conditional branches and surrounding assignments with arithmetic, boolean value producing comparison instructions, and conditional move instructions. In the very last invocation after reload, it will generate predicated instructions when supported by the target. The pass is located in `ifcvt.c`.

- Web construction

  This pass splits independent uses of each pseudo-register. This can improve effect of the other transformation, such as CSE or register allocation. Its source files are `web.c`.

- Life analysis

  This pass computes which pseudo-registers are live at each point in the program, and makes the first instruction that uses a value point at the instruction that computed the value. It then deletes computations whose results are never used, and combines memory references with add or subtract instructions to make autoincrement or autodecrement addressing. The pass is located in `flow.c`.

- Instruction combination

  This pass attempts to combine groups of two or three instructions that are related by data flow into single instructions. It combines the RTL expressions for the instructions by substitution, simplifies the result using algebra, and then attempts to match the result against the machine description. The pass is located in `combine.c`.

- Register movement

  This pass looks for cases where matching constraints would force an instruction to need a reload, and this reload would be a register-to-register move. It then attempts to change the registers used by the instruction to avoid the move instruction. The pass is located in `regmove.c`.

- Optimize mode switching

  This pass looks for instructions that require the processor to be in a specific “mode” and minimizes the number of mode changes required to satisfy all users. What these modes are, and what they apply to are completely target-specific. The source is located in `mode-switching.c`.

- Modulo scheduling

  This pass looks at innermost loops and reorders their instructions by overlapping different iterations. Modulo scheduling is performed immediately before instruction scheduling. The pass is located in (`modulo-sched.c`).

- Instruction scheduling

  This pass looks for instructions whose output will not be available by the time that it is used in subsequent instructions. Memory loads and floating point instructions often have this behavior on RISC machines. It re-orders instructions within a basic block to try to separate the definition and use of items that otherwise would cause pipeline stalls. This pass is performed twice, before and after register allocation. The pass is located in `haifa-sched.c`, `sched-deps.c`, `sched-ebb.c`, `sched-rgn.c` and `sched-vis.c`.

- Register allocation

  These passes make sure that all occurrences of pseudo registers are eliminated, either by allocating them to a hard register, replacing them by an equivalent expression (e.g. a constant) or by placing them on the stack. This is done in several subpasses:

  - Register class preferencing. The RTL code is scanned to find out which register class is best for each pseudo register. The source file is `regclass.c`.
  - Local register allocation. This pass allocates hard registers to pseudo registers that are used only within one basic block. Because the basic block is linear, it can use fast and powerful techniques to do a decent job. The source is located in `local-alloc.c`.
  - Global register allocation. This pass allocates hard registers for the remaining pseudo registers (those whose life spans are not contained in one basic block). The pass is located in `global.c`.

  - Reloading. This pass renumbers pseudo registers with the hardware registers numbers they were allocated. Pseudo registers that did not get hard registers are replaced with stack slots. Then it finds instructions that are invalid because a value has failed to end up in a register, or has ended up in a register of the wrong kind. It fixes up these instructions by reloading the problematical values temporarily into registers. Additional instructions are generated to do the copying.

    The reload pass also optionally eliminates the frame pointer and inserts instructions to save and restore call-clobbered registers around calls.

    Source files are `reload.c` and `reload1.c`, plus the header `reload.h` used for communication between them.

- Basic block reordering

  This pass implements profile guided code positioning. If profile information is not available, various types of static analysis are performed to make the predictions normally coming from the profile feedback (IE execution frequency, branch probability, etc). It is implemented in the file `bb-reorder.c`, and the various prediction routines are in `predict.c`.

- Variable tracking

  This pass computes where the variables are stored at each position in code and generates notes describing the variable locations to RTL code. The location lists are then generated according to these notes to debug information if the debugging information format supports location lists.

- Delayed branch scheduling

  This optional pass attempts to find instructions that can go into the delay slots of other instructions, usually jumps and calls. The source file name is `reorg.c`.

- Branch shortening

  On many RISC machines, branch instructions have a limited range. Thus, longer sequences of instructions must be used for long branches. In this pass, the compiler figures out what how far each instruction will be from each other instruction, and therefore whether the usual instructions, or the longer sequences, must be used for each branch.

- Register-to-stack conversion

  Conversion from usage of some hard registers to usage of a register stack may be done at this point. Currently, this is supported only for the floating-point registers of the Intel 80387 coprocessor. The source file name is `reg-stack.c`.

- Final

  This pass outputs the assembler code for the function. The source files are `final.c` plus `insn-output.c`; the latter is generated automatically from the machine description by the tool `genoutput`. The header file `conditions.h` is used for communication between these files. If mudflap is enabled, the queue of deferred declarations and any addressed constants (e.g., string literals) is processed by `mudflap_finish_file` into a synthetic constructor function containing calls into the mudflap runtime.

- Debugging information output

  This is run after final because it must output the stack slot offsets for pseudo registers that did not get hard registers. Source files are `dbxout.c` for DBX symbol table format, `sdbout.c` for SDB symbol table format, `dwarfout.c` for DWARF symbol table format, files `dwarf2out.c` and `dwarf2asm.c` for DWARF2 symbol table format, and `vmsdbgout.c` for VMS debug symbol table format.