On Tue, Apr 5, 2011 at 2:20 AM, Tom Stellard <tstel...@gmail.com> wrote:
> This pass removes conditions from conditional assignments when possible.
> This pass is useful for hardware that requires a lot of lowering passes
> that generate many CMP instructions.
> ---
> src/mesa/program/prog_optimize.c | 87
> ++++++++++++++++++++++++++++++++++++++
> 1 files changed, 87 insertions(+), 0 deletions(-)
>
> diff --git a/src/mesa/program/prog_optimize.c
> b/src/mesa/program/prog_optimize.c
> index f62d890..cf579d5 100644
> --- a/src/mesa/program/prog_optimize.c
> +++ b/src/mesa/program/prog_optimize.c
> @@ -1236,6 +1236,92 @@ print_it(struct gl_context *ctx, struct gl_program
> *program, const char *txt) {
> }
> #endif
>
> +/**
> + * This pass removes the condition from conditional assignments by converting
> + * CMP instructions to MOV instructions in the case where the CMP instruction
> + * is the first instruction to write to its destination register.
> + *
> + * There are two scenarios when the condition can be safely removed from a
> + * conditional assignment. The first is:
> + * int a;
> + *
> + * if (cond)
> + * a = foo;
> + *
> + * If 'a' is used after this when 'cond' is false, the behavior is
> + * undefined, and if we set 'a' to 'foo' when 'cond' is false, we haven't
> + * changed the behavior of the program.
> + *
> + * The second scenario is:
> + * int a;
> + *
> + * if (cond)
> + * a = foo;
> + * else
> + * a = bar;
> + *
> + * This is equivalent to these two conditional assignments:
> + * if (cond) a = foo;
> + * if (!cond) a = bar;
> + *
> + * In this case, it is safe to set 'a' to 'foo' when 'cond' is false, because
> + * 'a' will be overwritten with the value of 'bar' by the second conditional.
> + */
> +static void
> +_mesa_simplify_cmp(struct gl_program * program)
> +{
> + GLuint tempWrites[REG_ALLOCATE_MAX_PROGRAM_TEMPS];
> + GLuint outputWrites[MAX_PROGRAM_OUTPUTS];
> + GLuint i;
> +
> + if (dbg) {
> + printf("Optimize: Begin reads without writes\n");
> + _mesa_print_program(program);
> + }
> +
> + for (i = 0; i < REG_ALLOCATE_MAX_PROGRAM_TEMPS; i++) {
> + tempWrites[i] = 0;
> + }
> +
> + for (i = 0; i < MAX_PROGRAM_OUTPUTS; i++) {
> + outputWrites[i] = 0;
> + }
> +
> + for (i = 0; i < program->NumInstructions; i++) {
> + struct prog_instruction *inst = program->Instructions + i;
> + GLuint prevWriteMask;
> +
> + if (_mesa_is_flow_control_opcode(inst->Opcode) ||
> inst->DstReg.RelAddr) {
> + return;
> + }
> +
> + if (inst->DstReg.File == PROGRAM_OUTPUT) {
> + assert(inst->DstReg.Index < MAX_PROGRAM_OUTPUTS);
> + prevWriteMask = outputWrites[inst->DstReg.Index];
> + outputWrites[inst->DstReg.Index] |= inst->DstReg.WriteMask;
> + } else if (inst->DstReg.File == PROGRAM_TEMPORARY) {
> + assert(inst->DstReg.Index < REG_ALLOCATE_MAX_PROGRAM_TEMPS);
> + prevWriteMask = tempWrites[inst->DstReg.Index];
> + tempWrites[inst->DstReg.Index] |= inst->DstReg.WriteMask;
> + }
> +
> + /* For a CMP to be considered a conditional write, the destination
> + * register and source register two must be the same. */
> + if (inst->Opcode == OPCODE_CMP
> + && !(inst->DstReg.WriteMask & prevWriteMask)
> + && inst->SrcReg[2].File == inst->DstReg.File
> + && inst->SrcReg[2].Index == inst->DstReg.Index
> + && inst->DstReg.WriteMask == get_src_arg_mask(inst, 2, NO_MASK)) {
> +
> + inst->Opcode = OPCODE_MOV;
> + inst->SrcReg[0] = inst->SrcReg[1];
> + }
> + }
> + if (dbg) {
> + printf("Optimize: End reads without writes\n");
> + _mesa_print_program(program);
> + }
> +}
>
> /**
> * Apply optimizations to the given program to eliminate unnecessary
> @@ -1246,6 +1332,7 @@ _mesa_optimize_program(struct gl_context *ctx, struct
> gl_program *program)
> {
> GLboolean any_change;
>
> + _mesa_simplify_cmp(program);
> /* Stop when no modifications were output */
> do {
> any_change = GL_FALSE;
> --
> 1.7.3.4
>
The comment isn't very accurate on what the code does (comment is
about GLSL construct not ARB shader asm one). I think a comment
explaining what pattern the code replace would be helpful. If i am not
mistaken :
CMP A, x, y, A replaced MOV A, y
when A was not previoulsy written/modified in any previous instruction.
Otherwise this look good as it bail if there is control flow stuff
(maybe adding a comment to make sure no one remove the control flow
test).
Cheers,
Jerome
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