combine: Do not combine moves from hard registers

Programming / Compilers / GCC - segher [138bc75d-0d04-0410-961f-82ee72b054a4] - 22 October 2018 20:23 EDT

On most targets every function starts with moves from the parameter passing (hard) registers into pseudos. Similarly, after every call there is a move from the return register into a pseudo. These moves usually combine with later instructions (leaving pretty much the same instruction, just with a hard reg instead of a pseudo).

This isn't a good idea. Register allocation can get rid of unnecessary moves just fine, and moving the parameter passing registers into many later instructions tends to prevent good register allocation. This patch disallows combining moves from a hard (non-fixed) register.

This also avoid the problem mentioned in PR87600 #c3 (combining hard registers into inline assembler is problematic).

Because the register move can often be combined with other instructions
*itself*, for example for setting some condition code, this patch adds extra copies via new pseudos after every copy-from-hard-reg.

On some targets this reduces average code size. On others it increases it a bit, 0.1% or 0.2% or so. (I tested this on all *-linux targets).


PR rtl-optimization/87600
- combine.c: Add include of expr.h. (cant_combine_insn_p): Do not combine moves from any hard non-fixed register to a pseudo. (make_more_copies): New function, add a copy to a new pseudo after the moves from hard registers into pseudos. (rest_of_handle_combine): Declare rebuild_jump_labels_after_combine later. Call make_more_copies.

171920e88fe combine: Do not combine moves from hard registers
gcc/ChangeLog | 11 +++++++++++
gcc/combine.c | 50 ++++++++++++++++++++++++++++++++++++++++++++++----
2 files changed, 57 insertions(+), 4 deletions(-)

Upstream: gcc.gnu.org


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