[PATCH v5] RISC-V: Add small loop unrolling support via -munroll-only-small-loops

This patch introduces a conservative loop unrolling heuristic for the
RISC-V backend, controlled by the new -munroll-only-small-loops option.

When -munroll-only-small-loops is active, only small loops (those whose
body does not exceed a per-tune instruction threshold) are unrolled, and
the unroll factor is capped by a per-tune parameter, so that tight loops
benefit from unrolling without exposing larger loops to its code-size and
instruction-cache costs.

Mirroring the i386 model, -funroll-loops and -munroll-only-small-loops are
enabled by default at -O2 and above (for speed).  An explicit -funroll-loops
(or -funroll-all-loops) turns -munroll-only-small-loops off, so a user who
explicitly asks for unrolling still gets loops of any size unrolled.  The
size threshold and the maximum unroll factor come from the -mtune tuning
parameters.

Since -funroll-loops is now enabled by default at -O2 and above, a number
of pre-existing RISC-V tests that scan for a specific code sequence change
their output once loops are unrolled.  As those tests do not aim to exercise
unrolling, add -fno-unroll-loops to their options so they keep validating
their intended property regardless of the unrolling decision.

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_option_optimization_table):
	Enable -funroll-loops and -munroll-only-small-loops at -O2 and above.
	* config/riscv/riscv.cc (struct riscv_tune_param): Add fields
	small_loop_unroll_ninsns and small_loop_unroll_factor.
	(xt_c9501_tune_info): Set them.
	(riscv_loop_unroll_adjust): New function.
	(riscv_option_override): Turn off -munroll-only-small-loops when
	-funroll-loops or -funroll-all-loops is explicitly given.
	(TARGET_LOOP_UNROLL_ADJUST): Define.
	* config/riscv/riscv.opt (munroll-only-small-loops): New option.
	* config/riscv/riscv.opt.urls: Regenerate.
	* doc/invoke.texi (RISC-V Options): Document -munroll-only-small-loops.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/unroll-explicit.c: New test.
	* gcc.target/riscv/unroll-large-loop.c: New test.
	* gcc.target/riscv/unroll-pragma.c: New test.
	* gcc.target/riscv/unroll-small-loop-tune.c: New test.
	* gcc.target/riscv/unroll-small-loop.c: New test.
	* g++.dg/tree-prof/pr57451.C: Add -fno-unroll-loops.
	* g++.target/riscv/pr97682.C: Likewise.
	* gcc.dg/tree-ssa/pr117973-1.c: Likewise.
	* gcc.dg/vect/vect-alias-check-4.c: Likewise.
	* gcc.target/riscv/pr67731.c: Likewise.
	* gcc.target/riscv/rvv/autovec/pr113206-1.c: Likewise.
	* gcc.target/riscv/rvv/autovec/unop/math-nearbyint-0.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-1u.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-2u.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-3u.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls-vlmax/vec_extract-4u.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-1.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-2.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-3.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-4.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-5.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-6.c: Likewise.
	* gcc.target/riscv/rvv/autovec/vls/calling-convention-7.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-39.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-41.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-45.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-46.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-48.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-49.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-50.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-52.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-56.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-59.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-60.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-61.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-62.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-64.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-65.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-66.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-67.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-68.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-69.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-70.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-dynamic-frm-71.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-frm-insert-4.c: Likewise.
	* gcc.target/riscv/rvv/base/float-point-frm-insert-5.c: Likewise.
	* gcc.target/riscv/rvv/base/pr114352-3.c: Likewise.
	* gcc.target/riscv/rvv/base/scalar_move-7.c: Likewise.
	* gcc.target/riscv/rvv/base/spill-10.c: Likewise.
	* gcc.target/riscv/rvv/base/spill-11.c: Likewise.
	* gcc.target/riscv/rvv/base/spill-12.c: Likewise.
	* gcc.target/riscv/rvv/base/spill-8.c: Likewise.
	* gcc.target/riscv/rvv/base/spill-9.c: Likewise.
	* gcc.target/riscv/rvv/base/vwaddsub-1.c: Likewise.
	* gcc.target/riscv/rvv/base/zvbb_vandn_vx_constraint.c: Likewise.
	* gcc.target/riscv/rvv/base/zvbc_vx_constraint-2.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/avl_multiple-3.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/avl_multiple-4.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/avl_single-4.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/avl_single-6.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/avl_single-66.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/imm_loop_invariant-1.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/imm_loop_invariant-2.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/imm_loop_invariant-3.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/imm_loop_invariant-4.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/imm_loop_invariant-5.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/imm_loop_invariant-6.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-1.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-10.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-14.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-15.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-16.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-17.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-18.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-19.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-20.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-21.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-22.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-23.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-25.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-27.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-28.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-29.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-3.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-30.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-31.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-35.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-37.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-7.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-8.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_back_prop-9.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-17.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-18.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-19.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_bb_prop-20.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_conflict-11.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_switch_vtype-14.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_switch_vtype-15.c: Likewise.
	* gcc.target/riscv/rvv/vsetvl/vlmax_switch_vtype-16.c: Likewise.
	* gcc.target/riscv/slt-1.c: Likewise.
	* gcc.target/riscv/struct_vect_24.c: Likewise.