src/gallium/drivers/r600/sb/notes.markdown


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    Contents⊕
  
  
  r600-sb
  
  Debugging
  
  Environment variables
  Regression debugging
  
  Intermediate
  Representation
  
  Values
  Nodes
  Indirect addressing
  
  Passes
  References
  
  
r600-sb

Debugging
Environment variables

R600_DEBUG
There are new flags:

nosb - Disable sb backend for graphics shaders
sbcl - Enable optimization of compute shaders
(experimental)
sbdry - Dry run, optimize but use source bytecode -
useful if you only want to check shader dumps without the risk of
lockups and other problems
sbstat - Print optimization statistics (only time
so far)
sbdump - Print IR after some passes.
sbnofallback - Abort on errors instead of
fallback
sbdisasm - Use sb disassembler for shader
dumps
sbsafemath - Disable unsafe math optimizations


Regression debugging
If there are any regressions as compared to the default backend
(R600_SB=0), it's possible to use the following environment variables to
find the incorrectly optimized shader that causes the regression.

R600_SB_DSKIP_MODE - allows to skip optimization
for some shaders

0 - disabled (default)
1 - skip optimization for the shaders in the range
[R600_SB_DSKIP_START; R600_SB_DSKIP_END], that is, optimize only the
shaders that are not in this range
2 - optimize only the shaders in the range [R600_SB_DSKIP_START;
R600_SB_DSKIP_END]

R600_SB_DSKIP_START - start of the range
(1-based)
R600_SB_DSKIP_END - end of the range
(1-based)

Example - optimize only the shaders 5, 6, and 7:
R600_SB_DSKIP_START=5 R600_SB_DSKIP_END=7 R600_SB_DSKIP_MODE=2
All shaders compiled by the application are numbered starting from 1,
the number of shaders used by the application may be obtained by running
it with "R600_DEBUG=sb,sbstat" - it will print "sb: shader #index#" for
each compiled shader.
After figuring out the total number of shaders used by the
application, the variables above allow to use bisection to find the
shader that is the cause of regression. E.g. if the application uses 100
shaders, we can divide the range [1; 100] and run the application with
the optimization enabled only for the first half of the shaders:
R600_SB_DSKIP_START=1 R600_SB_DSKIP_END=50 R600_SB_DSKIP_MODE=2 <app>
If the regression is reproduced with these parameters, then the
failing shader is in the range [1; 50], if it's not reproduced - then
it's in the range [51; 100]. Then we can divide the new range again and
repeat the testing, until we'll reduce the range to a single failing
shader.
NOTE: This method relies on the assumption that the application
produces the same sequence of the shaders on each run. It's not always
true - some applications may produce different sequences of the shaders,
in such cases the tools like apitrace may be used to record the trace
with the application, then this method may be applied when replaying the
trace - also this may be faster and/or more convenient than testing the
application itself.

Intermediate Representation
Values
All kinds of the operands (literal constants, references to kcache
constants, references to GPRs, etc) are currently represented by the
value class (possibly it makes sense to switch to
hierarchy of classes derived from value instead, to
save some memory).
All values (except some pseudo values like the exec_mask or predicate
register) represent 32bit scalar values - there are no vector values,
CF/FETCH instructions use groups of 4 values for src and dst
operands.
Nodes
Shader programs are represented using the tree data structure, some
nodes contain a list of subnodes.
Control flow nodes
Control flow information is represented using four special node types
(based on the ideas from [1] )

region_node - single-entry, single-exit
region.
All loops and if's in the program are enclosed in region nodes.
Region nodes have two containers for phi nodes - region_node::loop_phi
contains the phi expressions to be executed at the region entry,
region_node::phi contains the phi expressions to be executed at the
region exit. It's the only type of the node that contains associated phi
expressions.
depart_node - "depart region $id after { ...
}"
Depart target region (jump to exit point) after executing contained
code.
repeat_node - "repeat region $id after { ...
}"
Repeat target region (jump to entry point) after executing contained
code.
if_node - "if (cond) { ... }"
Execute contained code if condition is true. The difference from [1] is that we don't have associated phi
expressions for the if_node, we enclose
if_node in the region_node and store
corresponding phi's in the region_node, this allows
more uniform handling.

The target region of depart and repeat nodes is always the region
where they are located (possibly in the nested region), there are no
arbitrary jumps/goto's - control flow in the program is always
structured.
Typical control flow constructs can be represented as in the
following examples:
GLSL:
if (cond) {
    < 1 >
} else {
    < 2 >
}
IR:
region #0 {
    depart region #0 after {
        if (cond) {
            depart region #0 after {
                < 1 >
            }
        }
        < 2 >
    }
    <region #0 phi nodes >
}
GLSL:
while (cond) {
    < 1 >
}
IR:
region #0 {
    <region #0 loop_phi nodes>
    repeat region #0 after {
        region #1 {
            depart region #1 after {
                if (!cond) {
                    depart region #0
                }
            }
        }
        < 1 >
    }
    <region #0 phi nodes>
}
'Break' and 'continue' inside the loops are directly translated to
the depart and repeat nodes for the corresponding loop region.
This may look a bit too complicated, but in fact this allows more
simple and uniform handling of the control flow.
All loop_phi and phi nodes for some region always have the same
number of source operands. The number of source operands for
region_node::loop_phi nodes is 1 + number of repeat nodes that reference
this region as a target. The number of source operands for
region_node::phi nodes is equal to the number of depart nodes that
reference this region as a target. All depart/repeat nodes for the
region have unique indices equal to the index of source operand for
phi/loop_phi nodes.
First source operand for region_node::loop_phi nodes (src[0]) is an
incoming value that enters the region from the outside. Each remaining
source operand comes from the corresponding repeat node.
More complex example:
GLSL:
a = 1;
while (a < 5) {
    a = a * 2;
    if (b == 3) {
        continue;
    } else {
        a = 6;
    }
    if (c == 4)
        break;
    a = a + 1;
}
IR with SSA form:
a.1 = 1;
region #0 {
    // loop phi values: src[0] - incoming, src[1] - from repeat_1, src[2] - from repeat_2
    region#0 loop_phi: a.2 = phi a.1, a.6, a.3

    repeat_1 region #0 after {
        a.3 = a.2 * 2;
        cond1 = (b == 3);
        region #1 {
            depart_0 region #1 after {
                if (cond1) {
                    repeat_2 region #0;
                }
            }
            a.4 = 6;

            region #1 phi: a.5 = phi a.4; // src[0] - from depart_0
        }
        cond2 = (c == 4);
        region #2 {
            depart_0 region #2 after {
                if (cond2) {
                    depart_0 region #0;
                }
            }
        }
        a.6 = a.5 + 1;
    }

    region #0 phi: a.7 = phi a.5 // src[0] from depart_0
}
Phi nodes with single source operand are just copies, they are not
really necessary, but this allows to handle all
depart_nodes in the uniform way.
Instruction nodes
Instruction nodes represent different kinds of instructions -
alu_node, cf_node,
fetch_node, etc. Each of them contains the "bc"
structure where all fields of the bytecode are stored (the type is
bc_alu for alu_node, etc). The
operands are represented using the vectors of pointers to
value class (node::src, node::dst)
SSA-specific nodes
Phi nodes currently don't have special node class, they are stored as
node. Destination vector contains a single destination
value, source vector contains 1 or more source values.
Psi nodes [5], [6] also don't have a
special node class and stored as node. Source vector
contains 3 values for each source operand - the value
of predicate, value of corresponding PRED_SEL field,
and the source value itself.
Indirect addressing
Special kind of values (VLK_RELREG) is used to represent indirect
operands. These values don't have SSA versions. The representation is
mostly based on the [2]. Indirect operand
contains the "offset/address" value (value::rel), (e.g. some SSA version
of the AR register value, though after some passes it may be any value -
constant, register, etc), also it contains the maydef and mayuse vectors
of pointers to values (similar to dst/src vectors in
the node) to represent the effects of aliasing in the
SSA form.
E.g. if we have the array R5.x ... R8.x and the following instruction
:
MOV R0.x, R[5 + AR].x
then source indirect operand is represented with the VLK_RELREG
value, value::rel is AR, value::maydef is empty (in fact it always
contain the same number of elements as mayuse to simplify the handling,
but they are NULLs), value::mayuse contains [R5.x, R6.x, R7.x, R8.x] (or
the corresponding SSA versions after ssa_rename).
Additional "virtual variables" as in HSSA
[2] are not used, also there is no special handling for "zero
versions". Typical programs in our case are small, indirect addressing
is rare, array sizes are limited by max gpr number, so we don't really
need to use special tricks to avoid the explosion of value versions.
Also this allows more precise liveness computation for array elements
without modifications to the algorithms.
With the following instruction:
MOV R[5+AR].x, R0.x
we'll have both maydef and mayuse vectors for dst operand filled with
array values initially: [R5.x, R6.x, R7.x, R8.x]. After the ssa_rename
pass mayuse will contain previous versions, maydef will contain new
potentially-defined versions.

Passes

bc_parser - creates the IR from the source
bytecode, initializes src and dst value vectors for instruction nodes.
Most ALU nodes have one dst operand and the number of source operands is
equal to the number of source operands for the ISA instruction. Nodes
for PREDSETxx instructions have 3 dst operands - dst[0] is dst gpr as in
the original instruction, other two are pseudo-operands that represent
possibly updated predicate and exec_mask. Predicate values are used in
the predicated alu instructions (node::pred), exec_mask values are used
in the if_nodes (if_node::cond). Each vector operand in the CF/TEX/VTX
instructions is represented with 4 values - components of the
vector.
ssa_prepare - creates phi expressions.
ssa_rename - renames the values (assigns
versions).
liveness - liveness computation, sets 'dead'
flag for unused nodes and values, optionally computes interference
information for the values.
dce_cleanup - eliminates 'dead' nodes, also
removes some unnecessary nodes created by bc_parser, e.g. the nodes for
the JUMP instructions in the source, containers for ALU groups (they
were only needed for the ssa_rename pass)
if_conversion - converts control flow with
if_nodes to the data flow in cases where it can improve performance
(small alu-only branches). Both branches are executed speculatively and
the phi expressions are replaced with conditional moves (CNDxx) to
select the final value using the same condition predicate as was used by
the original if_node. E.g. if_node used dst[2] from
PREDSETxx instruction, CNDxx now uses dst[0] from the same PREDSETxx
instruction.
peephole - peephole optimizations
gvn - Global Value Numbering [2], [3]
gcm - Global Code Motion [3]. Also performs grouping of the instructions
of the same kind (CF/FETCH/ALU).
register allocation passes, some ideas are used from [4], but implementation is simplified to make it
more efficient in terms of the compilation speed (e.g. no recursive
recoloring) while achieving good enough results.

ra_split - prepares the program to register
allocation. Splits live ranges for constrained values by inserting the
copies to/from temporary values, so that the live range of the
constrained values becomes minimal.
ra_coalesce - performs global allocation on
registers used in CF/FETCH instructions. It's performed first to make
sure they end up in the same GPR. Also tries to allocate all values
involved in copies (inserted by the ra_split pass) to the same register,
so that the copies may be eliminated.
ra_init - allocates gpr arrays (if indirect
addressing is used), and remaining values.

post_scheduler - ALU scheduler, handles VLIW
packing and performs the final register allocation for local values
inside ALU clauses. Eliminates all coalesced copies (if src and dst of
the copy are allocated to the same register).
ra_checker - optional debugging pass that tries
to catch basic errors of the scheduler or regalloc,
bc_finalize - propagates the regalloc
information from values in node::src and node::dst vectors to the
bytecode fields, converts control flow structure (region/depart/repeat)
to the target instructions (JUMP/ELSE/POP,
LOOP_START/LOOP_END/LOOP_CONTINUE/LOOP_BREAK).
bc_builder - builds final bytecode,


References
[1] "Tree-Based
Code Optimization. A Thesis Proposal", Carl McConnell
[2] "Effective
Representation of Aliases and Indirect Memory Operations in SSA Form",
Fred Chow, Sun Chan, Shin-Ming Liu, Raymond Lo, Mark Streich
[3] "Global
Code Motion. Global Value Numbering.", Cliff Click
[4] "Register
Allocation for Programs in SSA Form", Sebastian Hack
[5] "An
extension to the SSA representation for predicated code", Francois de
Ferriere
[6] "Improvements
to the Psi-SSA Representation", F. de Ferriere