On Mon, Jun 4, 2012 at 3:45 PM, William J. Schmidt
<wschm...@linux.vnet.ibm.com> wrote:
> Hi Richard,
>
> Here's a revision of the hoist-adjacent-loads patch. I'm sorry for the
> delay since the last revision, but my performance testing has been
> blocked waiting for a fix to PR53487. I ended up applying a test
> version of the patch to 4.7 and ran performance numbers with that
> instead, with no degradations.
>
> In addition to addressing your comments, this patch contains one bug fix
> where local_mem_dependence was called on the wrong blocks after swapping
> def1 and def2.
>
> Bootstrapped with no regressions on powerpc64-unknown-linux-gnu. Is
> this version ok for trunk? I won't commit it until I can do final
> testing on trunk in conjunction with a fix for PR53487.
>
> Thanks,
> Bill
>
>
> 2012-06-04 Bill Schmidt <wschm...@linux.vnet.ibm.com>
>
> * opts.c: Add -fhoist_adjacent_loads to -O2 and above.
> * tree-ssa-phiopt.c (tree_ssa_phiopt_worker): Add argument to forward
> declaration.
> (hoist_adjacent_loads, gate_hoist_loads): New forward declarations.
> (tree_ssa_phiopt): Call gate_hoist_loads.
> (tree_ssa_cs_elim): Add parm to tree_ssa_phiopt_worker call.
> (tree_ssa_phiopt_worker): Add do_hoist_loads to formal arg list; call
> hoist_adjacent_loads.
> (local_mem_dependence): New function.
> (hoist_adjacent_loads): Likewise.
> (gate_hoist_loads): Likewise.
> * common.opt (fhoist-adjacent-loads): New switch.
> * Makefile.in (tree-ssa-phiopt.o): Added dependencies.
> * params.def (PARAM_MIN_CMOVE_STRUCT_ALIGN): New param.
>
>
> Index: gcc/opts.c
> ===================================================================
> --- gcc/opts.c (revision 187805)
> +++ gcc/opts.c (working copy)
> @@ -489,6 +489,7 @@ static const struct default_options default_option
> { OPT_LEVELS_2_PLUS, OPT_falign_functions, NULL, 1 },
> { OPT_LEVELS_2_PLUS, OPT_ftree_tail_merge, NULL, 1 },
> { OPT_LEVELS_2_PLUS_SPEED_ONLY, OPT_foptimize_strlen, NULL, 1 },
> + { OPT_LEVELS_2_PLUS, OPT_fhoist_adjacent_loads, NULL, 1 },
>
> /* -O3 optimizations. */
> { OPT_LEVELS_3_PLUS, OPT_ftree_loop_distribute_patterns, NULL, 1 },
> Index: gcc/tree-ssa-phiopt.c
> ===================================================================
> --- gcc/tree-ssa-phiopt.c (revision 187805)
> +++ gcc/tree-ssa-phiopt.c (working copy)
> @@ -37,9 +37,17 @@ along with GCC; see the file COPYING3. If not see
> #include "cfgloop.h"
> #include "tree-data-ref.h"
> #include "tree-pretty-print.h"
> +#include "gimple-pretty-print.h"
> +#include "insn-config.h"
> +#include "expr.h"
> +#include "optabs.h"
>
> +#ifndef HAVE_conditional_move
> +#define HAVE_conditional_move (0)
> +#endif
> +
> static unsigned int tree_ssa_phiopt (void);
> -static unsigned int tree_ssa_phiopt_worker (bool);
> +static unsigned int tree_ssa_phiopt_worker (bool, bool);
> static bool conditional_replacement (basic_block, basic_block,
> edge, edge, gimple, tree, tree);
> static int value_replacement (basic_block, basic_block,
> @@ -53,6 +61,9 @@ static bool cond_store_replacement (basic_block, b
> static bool cond_if_else_store_replacement (basic_block, basic_block,
> basic_block);
> static struct pointer_set_t * get_non_trapping (void);
> static void replace_phi_edge_with_variable (basic_block, edge, gimple, tree);
> +static void hoist_adjacent_loads (basic_block, basic_block,
> + basic_block, basic_block);
> +static bool gate_hoist_loads (void);
>
> /* This pass tries to replaces an if-then-else block with an
> assignment. We have four kinds of transformations. Some of these
> @@ -138,12 +149,56 @@ static void replace_phi_edge_with_variable (basic_
> bb2:
> x = PHI <x' (bb0), ...>;
>
> - A similar transformation is done for MAX_EXPR. */
> + A similar transformation is done for MAX_EXPR.
>
> +
> + This pass also performs a fifth transformation of a slightly different
> + flavor.
> +
> + Adjacent Load Hoisting
> + ----------------------
> +
> + This transformation replaces
> +
> + bb0:
> + if (...) goto bb2; else goto bb1;
> + bb1:
> + x1 = (<expr>).field1;
> + goto bb3;
> + bb2:
> + x2 = (<expr>).field2;
> + bb3:
> + # x = PHI <x1, x2>;
> +
> + with
> +
> + bb0:
> + x1 = (<expr>).field1;
> + x2 = (<expr>).field2;
> + if (...) goto bb2; else goto bb1;
> + bb1:
> + goto bb3;
> + bb2:
> + bb3:
> + # x = PHI <x1, x2>;
> +
> + The purpose of this transformation is to enable generation of conditional
> + move instructions such as Intel CMOVE or PowerPC ISEL. Because one of
> + the loads is speculative, the transformation is restricted to very
> + specific cases to avoid introducing a page fault. We are looking for
> + the common idiom:
> +
> + if (...)
> + x = y->left;
> + else
> + x = y->right;
> +
> + where left and right are typically adjacent pointers in a tree structure.
> */
> +
> static unsigned int
> tree_ssa_phiopt (void)
> {
> - return tree_ssa_phiopt_worker (false);
> + return tree_ssa_phiopt_worker (false, gate_hoist_loads ());
> }
>
> /* This pass tries to transform conditional stores into unconditional
> @@ -190,7 +245,7 @@ tree_ssa_phiopt (void)
> static unsigned int
> tree_ssa_cs_elim (void)
> {
> - return tree_ssa_phiopt_worker (true);
> + return tree_ssa_phiopt_worker (true, false);
> }
>
> /* Return the singleton PHI in the SEQ of PHIs for edges E0 and E1. */
> @@ -227,9 +282,11 @@ static tree condstoretemp;
> /* The core routine of conditional store replacement and normal
> phi optimizations. Both share much of the infrastructure in how
> to match applicable basic block patterns. DO_STORE_ELIM is true
> - when we want to do conditional store replacement, false otherwise. */
> + when we want to do conditional store replacement, false otherwise.
> + DO_HOIST_LOADS is true when we want to hoist adjacent loads out
> + of diamond control flow patterns, false otherwise. */
> static unsigned int
> -tree_ssa_phiopt_worker (bool do_store_elim)
> +tree_ssa_phiopt_worker (bool do_store_elim, bool do_hoist_loads)
> {
> basic_block bb;
> basic_block *bb_order;
> @@ -312,6 +369,21 @@ static unsigned int
> cfgchanged = true;
> continue;
> }
> + else if (do_hoist_loads
> + && EDGE_SUCC (bb1, 0)->dest == EDGE_SUCC (bb2, 0)->dest)
> + {
> + basic_block bb3 = EDGE_SUCC (bb1, 0)->dest;
> +
> + if (!FLOAT_TYPE_P (TREE_TYPE (gimple_cond_lhs (cond_stmt)))
> + && single_succ_p (bb1)
> + && single_succ_p (bb2)
> + && single_pred_p (bb1)
> + && single_pred_p (bb2)
> + && EDGE_COUNT (bb->succs) == 2
> + && EDGE_COUNT (bb3->preds) == 2)
> + hoist_adjacent_loads (bb, bb1, bb2, bb3);
> + continue;
> + }
> else
> continue;
>
> @@ -1707,6 +1779,207 @@ cond_if_else_store_replacement (basic_block then_b
> return ok;
> }
>
> +/* Return TRUE if STMT has a VUSE whose corresponding VDEF is in BB. */
> +
> +static bool
> +local_mem_dependence (gimple stmt, basic_block bb)
> +{
> + tree vuse = gimple_vuse (stmt);
> + gimple def;
> +
> + if (!vuse)
> + return false;
> +
> + def = SSA_NAME_DEF_STMT (vuse);
> + return (def && gimple_bb (def) == bb);
> +}
> +
> +/* Given a "diamond" control-flow pattern where BB0 tests a condition,
> + BB1 and BB2 are "then" and "else" blocks dependent on this test,
> + and BB3 rejoins control flow following BB1 and BB2, look for
> + opportunities to hoist loads as follows. If BB3 contains a PHI of
> + two loads, one each occurring in BB1 and BB2, and the loads are
> + provably of adjacent fields in the same structure, then move both
> + loads into BB0. Of course this can only be done if there are no
> + dependencies preventing such motion.
> +
> + One of the hoisted loads will always be speculative, so the
> + transformation is currently conservative:
> +
> + - The fields must be strictly adjacent.
> + - The two fields must occupy a single memory block that is
> + guaranteed to not cross a page boundary.
> +
> + The last is difficult to prove, as such memory blocks should be
> + aligned on the minimum of the stack alignment boundary and the
> + alignment guaranteed by heap allocation interfaces. Thus we rely
> + on a parameter for the alignment value.
> +
> + Provided a good value is used for the last case, the first
> + restriction could possibly be relaxed. */
> +
> +static void
> +hoist_adjacent_loads (basic_block bb0, basic_block bb1,
> + basic_block bb2, basic_block bb3)
> +{
> + int param_align = PARAM_VALUE (PARAM_MIN_CMOVE_STRUCT_ALIGN);
> + unsigned param_align_bits = (unsigned) (param_align * BITS_PER_UNIT);
> + gimple_stmt_iterator gsi;
> +
> + /* Walk the phis in bb3 looking for an opportunity. We are looking
> + for phis of two SSA names, one each of which is defined in bb1 and
> + bb2. */
> + for (gsi = gsi_start_phis (bb3); !gsi_end_p (gsi); gsi_next (&gsi))
> + {
> + gimple phi_stmt = gsi_stmt (gsi);
> + gimple def1, def2, defswap;
> + tree arg1, arg2, ref1, ref2, field1, field2, fieldswap;
> + tree tree_offset1, tree_offset2, tree_size2, next;
> + int offset1, offset2, size2;
> + unsigned align1;
> + gimple_stmt_iterator gsi2;
> + basic_block bb_for_def1, bb_for_def2;
> +
> + if (gimple_phi_num_args (phi_stmt) != 2)
> + continue;
> +
> + arg1 = gimple_phi_arg_def (phi_stmt, 0);
> + arg2 = gimple_phi_arg_def (phi_stmt, 1);
> +
> + if (TREE_CODE (arg1) != SSA_NAME
> + || TREE_CODE (arg2) != SSA_NAME
> + || SSA_NAME_IS_DEFAULT_DEF (arg1)
> + || SSA_NAME_IS_DEFAULT_DEF (arg2))
> + continue;
> +
> + def1 = SSA_NAME_DEF_STMT (arg1);
> + def2 = SSA_NAME_DEF_STMT (arg2);
> +
> + if ((gimple_bb (def1) != bb1 || gimple_bb (def2) != bb2)
> + && (gimple_bb (def2) != bb1 || gimple_bb (def1) != bb2))
> + continue;
> +
> + /* Check the mode of the arguments to be sure a conditional move
> + can be generated for it. */
> + if (!optab_handler (cmov_optab, TYPE_MODE (TREE_TYPE (arg1))))
> + continue;
> +
> + /* Both statements must be assignments whose RHS is a COMPONENT_REF.
> */
> + if (!gimple_assign_single_p (def1)
> + || !gimple_assign_single_p (def2))
> + continue;
> +
> + ref1 = gimple_assign_rhs1 (def1);
> + ref2 = gimple_assign_rhs1 (def2);
> +
> + if (TREE_CODE (ref1) != COMPONENT_REF
> + || TREE_CODE (ref2) != COMPONENT_REF)
> + continue;
> +
> + /* The zeroth operand of the two component references must be
> + identical. It is not sufficient to compare get_base_address of
> + the two references, because this could allow for different
> + elements of the same array in the two trees. It is not safe to
> + assume that the existence of one array element implies the
> + existence of a different one. */
> + if (!operand_equal_p (TREE_OPERAND (ref1, 0), TREE_OPERAND (ref2, 0),
> 0))
> + continue;
> +
> + field1 = TREE_OPERAND (ref1, 1);
> + field2 = TREE_OPERAND (ref2, 1);
> +
> + /* Check for field adjacency, and ensure field1 comes first. */
> + for (next = DECL_CHAIN (field1);
> + next && TREE_CODE (next) != FIELD_DECL;
> + next = DECL_CHAIN (next))
> + ;
> +
> + if (next != field2)
> + {
> + for (next = DECL_CHAIN (field2);
> + next && TREE_CODE (next) != FIELD_DECL;
> + next = DECL_CHAIN (next))
> + ;
> +
> + if (next != field1)
> + continue;
> +
> + fieldswap = field1;
> + field1 = field2;
> + field2 = fieldswap;
> + defswap = def1;
> + def1 = def2;
> + def2 = defswap;
> + /* Don't swap bb1 and bb2 as we may have more than one
> + phi to process successfully. */
> + bb_for_def1 = bb2;
> + bb_for_def2 = bb1;
> + }
> + else
> + {
> + bb_for_def1 = bb1;
> + bb_for_def2 = bb2;
> + }
> +
> + /* Check for proper alignment of the first field. */
> + tree_offset1 = bit_position (field1);
> + tree_offset2 = bit_position (field2);
> + tree_size2 = DECL_SIZE (field2);
> +
> + if (!host_integerp (tree_offset1, 1)
> + || !host_integerp (tree_offset2, 1)
> + || !host_integerp (tree_size2, 1))
> + continue;
> +
> + offset1 = TREE_INT_CST_LOW (tree_offset1);
> + offset2 = TREE_INT_CST_LOW (tree_offset2);
> + size2 = TREE_INT_CST_LOW (tree_size2);
> + align1 = DECL_ALIGN (field1) % param_align_bits;
> +
> + if (offset1 % BITS_PER_UNIT != 0)
> + continue;
> +
> + /* The two field references must fit within a block of memory that
> + is guaranteed to be on the same page. */
> + if (align1 + offset2 - offset1 + size2 > param_align_bits)
> + continue;
If param_align_bits were the cache-line size then this would be appropriate
for a check if the transform is profitable. But the comment suggests it
is a check for correctness? We do already have a l1-cache-line-size
param btw.
Did you check packed nested structs? Like
struct A {
int i;
int j;
};
struct {
char c;
struct A a;
} __attribute__((packed)) x;
and loads of x.a.i and x.a.j? I _think_ that DECL_ALIGN of the i and j
FIELD_DECL may still be 4 in this case. Which means that if you
are performing the check for correctness then it has to be a lot more
conservative.
I _think_ we have concluded that if you see loads of p->a and p->b
then either both trap or both do not trap, so the check for correctness
is that you see two COMPONENT_REFs of the same base for
adjacent fields.
Btw, you do not restrict the hoisting to the case where the the/else
block will be empty afterwards? It seems to be the transform is
not profitable if we end up doing the if() anyways - it might even
be not profitable (similar to the case where either path is very likely
and the other not - conditional moves are mostly profitable when
the possibility of the branches is equal).
Thanks,
Richard.
> +
> + /* The two expressions cannot be dependent upon vdefs defined
> + in bb1/bb2. */
> + if (local_mem_dependence (def1, bb_for_def1)
> + || local_mem_dependence (def2, bb_for_def2))
> + continue;
> +
> + /* The conditions are satisfied; hoist the loads from bb1 and bb2 into
> + bb0. We hoist the first one first so that a cache miss is handled
> + efficiently regardless of hardware cache-fill policy. */
> + gsi2 = gsi_for_stmt (def1);
> + gsi_move_to_bb_end (&gsi2, bb0);
> + gsi2 = gsi_for_stmt (def2);
> + gsi_move_to_bb_end (&gsi2, bb0);
> +
> + if (dump_file && (dump_flags & TDF_DETAILS))
> + {
> + fprintf (dump_file,
> + "\nHoisting adjacent loads from %d and %d into %d: \n",
> + bb_for_def1->index, bb_for_def2->index, bb0->index);
> + print_gimple_stmt (dump_file, def1, 0, TDF_VOPS|TDF_MEMSYMS);
> + print_gimple_stmt (dump_file, def2, 0, TDF_VOPS|TDF_MEMSYMS);
> + }
> + }
> +}
> +
> +/* Determine whether we should attempt to hoist adjacent loads out of
> + diamond patterns in pass_phiopt. Always hoist loads if
> + -fhoist-adjacent-loads is specified and the target machine has
> + a conditional move instruction. */
> +
> +static bool
> +gate_hoist_loads (void)
> +{
> + return (flag_hoist_adjacent_loads == 1 && HAVE_conditional_move);
> +}
> +
> /* Always do these optimizations if we have SSA
> trees to work on. */
> static bool
> Index: gcc/common.opt
> ===================================================================
> --- gcc/common.opt (revision 187805)
> +++ gcc/common.opt (working copy)
> @@ -1186,6 +1186,11 @@ fgraphite-identity
> Common Report Var(flag_graphite_identity) Optimization
> Enable Graphite Identity transformation
>
> +fhoist-adjacent-loads
> +Common Report Var(flag_hoist_adjacent_loads) Optimization
> +Enable hoisting adjacent loads to encourage generating conditional move
> +instructions
> +
> floop-parallelize-all
> Common Report Var(flag_loop_parallelize_all) Optimization
> Mark all loops as parallel
> Index: gcc/Makefile.in
> ===================================================================
> --- gcc/Makefile.in (revision 187805)
> +++ gcc/Makefile.in (working copy)
> @@ -2355,7 +2355,8 @@ tree-ssa-phiopt.o : tree-ssa-phiopt.c $(CONFIG_H)
> $(TM_H) $(GGC_H) $(TREE_H) $(TM_P_H) $(BASIC_BLOCK_H) \
> $(TREE_FLOW_H) $(TREE_PASS_H) $(TREE_DUMP_H) langhooks.h $(FLAGS_H) \
> $(DIAGNOSTIC_H) $(TIMEVAR_H) pointer-set.h domwalk.h $(CFGLOOP_H) \
> - $(TREE_DATA_REF_H) tree-pretty-print.h
> + $(TREE_DATA_REF_H) tree-pretty-print.h gimple-pretty-print.h \
> + insn-config.h $(EXPR_H) $(OPTABS_H)
> tree-nrv.o : tree-nrv.c $(CONFIG_H) $(SYSTEM_H) coretypes.h \
> $(TM_H) $(TREE_H) $(FUNCTION_H) $(BASIC_BLOCK_H) $(FLAGS_H) \
> $(DIAGNOSTIC_H) $(TREE_FLOW_H) $(TIMEVAR_H) $(TREE_DUMP_H) $(TREE_PASS_H) \
> Index: gcc/params.def
> ===================================================================
> --- gcc/params.def (revision 187805)
> +++ gcc/params.def (working copy)
> @@ -979,6 +979,13 @@ DEFPARAM (PARAM_MAX_TRACKED_STRLENS,
> "track string lengths",
> 1000, 0, 0)
>
> +/* For adjacent load hoisting transformation in tree-ssa-phiopt.c. */
> +DEFPARAM (PARAM_MIN_CMOVE_STRUCT_ALIGN,
> + "min-cmove-struct-align",
> + "Minimum byte alignment to assume for structures in the stack "
> + "or heap when considering load hoisting for conditional moves",
> + 32, 8, 256)
> +
> /*
> Local variables:
> mode:c
>
>
