The following briefly describes the rtl generation and optimization passes that are run after tree optimization.
The source files for RTL generation include
Also, the file
insn-emit.c, generated from the machine description by the
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
gencodes, tell this pass which standard names are available
for use and which patterns correspond to them.
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.
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.
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.
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.
This pass moves constant expressions out of loops, and optionally does strength-reduction as well. The pass is located in loop.c. Loop dependency analysis routines are contained in dependence.c. This pass is seriously out-of-date and is supposed to be replaced by a new one described below in near future.
A second loop optimization pass takes care of basic block level optimizations—unrolling, peeling and unswitching loops. The source files are cfgloopanal.c and cfgloopmanip.c containing generic loop analysis and manipulation code, loop-init.c with initialization and finalization code, loop-unswitch.c for loop unswitching and loop-unroll.c for loop unrolling and peeling. It also contains a separate loop invariant motion pass implemented in loop-invariant.c.
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.
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.
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.
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.
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.
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.
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.
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).
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.
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:
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.
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.
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.
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.
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.
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.
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.
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.