GCC Middle and Back End API Reference
Data Fields
phiprop_d Struct Reference

Data Fields

tree value
tree vuse

Detailed Description

Backward propagation of indirect loads through PHIs. Copyright (C) 2007-2013 Free Software Foundation, Inc. Contributed by Richard Guenther rguen.nosp@m.ther.nosp@m.@suse.nosp@m..de

This file is part of GCC.

GCC is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

GCC is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with GCC; see the file COPYING3. If not see http://www.gnu.org/licenses/. This pass propagates indirect loads through the PHI node for its address to make the load source possibly non-addressable and to allow for PHI optimization to trigger.

For example the pass changes

addr_1 = PHI <&a, &b>

tmp_1 = *addr_1;

to

tmp_1 = PHI <a, b>

but also handles more complex scenarios like

D.2077_2 = &this_1(D)->a1; ...

b_12 = PHI <&c(2), D.2077_2(3)>

D.2114_13 = *b_12; ...

b_15 = PHI <b_12(4), &b(5)>

D.2080_5 = &this_1(D)->a0; ...

b_18 = PHI <D.2080_5(6), &c(7)>

...

b_21 = PHI <b_15(8), b_18(9)>

D.2076_8 = *b_21;

where the addresses loaded are defined by PHIs itself. The above happens for

std::max(std::min(a0, c), std::min(std::max(a1, c), b))

where this pass transforms it to a form later PHI optimization recognizes and transforms it to the simple

D.2109_10 = this_1(D)->a1; D.2110_11 = c; D.2114_31 = MAX_EXPR <D.2109_10, D.2110_11>; D.2115_14 = b; D.2125_17 = MIN_EXPR <D.2115_14, D.2114_31>; D.2119_16 = this_1(D)->a0; D.2124_32 = MIN_EXPR <D.2110_11, D.2119_16>; D.2076_33 = MAX_EXPR <D.2125_17, D.2124_32>;

The pass does a dominator walk processing loads using a basic-block local analysis and stores the result for use by transformations on dominated basic-blocks. Structure to keep track of the value of a dereferenced PHI result and the virtual operand used for that dereference.

Field Documentation

tree phiprop_d::value
tree phiprop_d::vuse
The documentation for this struct was generated from the following file: