Loop unrolling will need to know a few more options in the future.
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>
Now that loop_controls no longer creates normatively bound loops,
there is no need for ir_loop::normative_bound or the
lower_bounded_loops pass.
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Previously, when loop_controls analyzed a loop and found that it had a
fixed bound (known at compile time), it would remove all of the loop
terminators and instead set the loop's normative_bound field to force
the loop to execute the correct number of times.
This made loop unrolling easy, but it had a serious disadvantage.
Since most GPU's don't have a native mechanism for executing a loop a
fixed number of times, in order to implement the normative bound, the
back-ends would have to synthesize a new loop induction variable. As
a result, many loops wound up having two induction variables instead
of one. This caused extra register pressure and unnecessary
instructions.
This patch modifies loop_controls so that it doesn't set the loop's
normative_bound anymore. Instead it leaves one of the terminators in
the loop (the limiting terminator), so the back-end doesn't have to go
to any extra work to ensure the loop terminates at the right time.
This complicates loop unrolling slightly: when deciding whether a loop
can be unrolled, we have to account for the presence of the limiting
terminator. And when we do unroll the loop, we have to remove the
limiting terminator first.
For an example of how this results in more efficient back end code,
consider the loop:
for (int i = 0; i < 100; i++) {
total += i;
}
Previous to this patch, on i965, this loop would compile down to this
(vec4) native code:
mov(8) g4<1>.xD 0D
mov(8) g8<1>.xD 0D
loop:
cmp.ge.f0(8) null g8<4;4,1>.xD 100D
(+f0) if(8)
break(8)
endif(8)
add(8) g5<1>.xD g5<4;4,1>.xD g4<4;4,1>.xD
add(8) g8<1>.xD g8<4;4,1>.xD 1D
add(8) g4<1>.xD g4<4;4,1>.xD 1D
while(8) loop
(notice that both g8 and g4 are loop induction variables; one is used
to terminate the loop, and the other is used to accumulate the total).
After this patch, the same loop compiles to:
mov(8) g4<1>.xD 0D
loop:
cmp.ge.f0(8) null g4<4;4,1>.xD 100D
(+f0) if(8)
break(8)
endif(8)
add(8) g5<1>.xD g5<4;4,1>.xD g4<4;4,1>.xD
add(8) g4<1>.xD g4<4;4,1>.xD 1D
while(8) loop
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
This value is now redundant with
loop_variable_state::limiting_terminator->iterations and
ir_loop::normative_bound.
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
The old logic of loop_unroll_visitor::visit_leave(ir_loop *) was:
heuristics to skip unrolling in various circumstances;
if (loop contains more than one jump)
return;
else if (loop contains one jump) {
if (the jump is an unconditional "break" at the end of the loop) {
remove the break and set iteration count to 1;
fall through to simple loop unrolling code;
} else {
for (each "if" statement in the loop body)
see if the jump is a "break" at the end of one of its forks;
if (the "break" wasn't found)
return;
splice the remainder of the loop into the other fork of the "if";
remove the "break";
complex loop unrolling code;
return;
}
}
simple loop unrolling code;
return;
These tasks have been moved to their own functions:
- splice the remainder of the loop into the other fork of the "if"
- simple loop unrolling code
- complex loop unrolling code
And the logic has been flattened to:
heuristics to skip unrolling in various circumstances;
if (loop contains more than one jump)
return;
if (loop contains no jumps) {
simple loop unroll;
return;
}
if (the jump is an unconditional "break" at the end of the loop) {
remove the break;
simple loop unroll with iteration count of 1;
return;
}
for (each "if" statement in the loop body) {
if (the jump is a "break" at the end of one of its forks) {
splice the remainder of the loop into the other fork of the "if";
remove the "break";
complex loop unroll;
return;
}
}
This will make it easier to modify the loop unrolling algorithm in a
future patch.
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
This gives the compiler the chance to inline and not export class symbols
even in the absence of LTO. Saves about 60kb on disk.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@.intel.com>
Before, we were only counting top-level instructions. But if we have
an assignment of a giant expression tree (such as the ones eventually
generated by glsl-fs-unroll), we were counting the same as an
assignment of a variable deref.
glsl-fs-unroll-explosion now fails in a reasonable amount of time on
i965 because the unrolling didn't go ridiculously far.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Avoid unrollong loops that are either nested loops or
where the loop body times the unroll count is huge.
The change is far from being perfect but it extends the
loop unrolling decision heuristic by some additional
safeguard. In particular this cuts down compilation of
a shader precomputing atmospheric scattering integral
tables containing two nesting levels in a loop from
something way beyond some minutes (I never waited for
it to finish) to some fractions of a second.
This fixes piglit tests glsl-fs-unroll-explosion and
glsl-vs-unroll-explosion on r600g.
Reviewed-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Currently we only unroll loops with conditional breaks at the end, which is
the form that lower_jumps generates.
However, if breaks are not lowered, they tend to appear at the beginning, so
add support for a conditional break anywhere.
Signed-off-by: Luca Barbieri <luca@luca-barbieri.com>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Found this bug by code inspection. Based off the comments just before
this code, the intent is to find whether the break exists in the "then"
branch or the "else" branch. However, the code actually looked at the
last instruction in the "then" branch twice.
If the loop ends with an if with one break or in a single break unroll
it. Loops that end with a continue will have that continue removed by
the redundant jump optimizer. Likewise loops that end with an
if-statement with a break at the end of both branches will have the
break pulled out after the if-statement.
Loops of the form
for (...) {
do_something1();
if (cond) {
do_something2();
break;
} else {
do_something3();
}
}
will be unrolled as
do_something1();
if (cond) {
do_something2();
} else {
do_something3();
do_something1();
if (cond) {
do_something2();
} else {
do_something3();
/* Repeat inserting iterations here.*/
}
}
ir_lower_jumps can guarantee that all loops are put in this form
and thus all loops are now potentially unrollable if an upper bound
on the number of iterations can be found.
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
This increases the chance that GLSL programs will actually work.
Note that continues and returns are not yet lowered, so linking
will just fail if not supported.
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
