[Mesa-dev] [PATCH] r600g/sb: fix stack size computation on evergreen

[Tom Stellard]

On Sat, Dec 07, 2013 at 07:06:36PM +0400, Vadim Girlin wrote:
> On evergreen we have to reserve 1 stack element in some additional cases
> besides the ones mentioned in the docs, but stack size computation was
> recently reimplemented exactly as described in the docs by the patch that
> added workarounds for stack issues on EG/CM, resulting in regressions
> with some apps (Serious Sam 3).
> 
> This patch fixes it by restoring previous behavior.
> 
> Fixes https://bugs.freedesktop.org/show_bug.cgi?id=72369
> 
> Signed-off-by: Vadim Girlin <vadimgirlin at gmail.com>
> Cc: "10.0" <mesa-stable at lists.freedesktop.org>
> ---
>  src/gallium/drivers/r600/sb/sb_bc_finalize.cpp | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)
> 
> diff --git a/src/gallium/drivers/r600/sb/sb_bc_finalize.cpp b/src/gallium/drivers/r600/sb/sb_bc_finalize.cpp
> index bc71cf8..355eb63 100644
> --- a/src/gallium/drivers/r600/sb/sb_bc_finalize.cpp
> +++ b/src/gallium/drivers/r600/sb/sb_bc_finalize.cpp
> @@ -770,7 +770,6 @@ void bc_finalizer::update_ngpr(unsigned gpr) {
>  unsigned bc_finalizer::get_stack_depth(node *n, unsigned &loops,
>                                             unsigned &ifs, unsigned add) {
>  	unsigned stack_elements = add;
> -	bool has_non_wqm_push_with_loops_on_stack = false;
>  	bool has_non_wqm_push = (add != 0);
>  	region_node *r = n->is_region() ?
>  			static_cast<region_node*>(n) : n->get_parent_region();
> @@ -781,8 +780,6 @@ unsigned bc_finalizer::get_stack_depth(node *n, unsigned &loops,
>  	while (r) {
>  		if (r->is_loop()) {
>  			++loops;
> -			if (has_non_wqm_push)
> -				has_non_wqm_push_with_loops_on_stack = true;
>  		} else {
>  			++ifs;
>  			has_non_wqm_push = true;
> @@ -795,15 +792,26 @@ unsigned bc_finalizer::get_stack_depth(node *n, unsigned &loops,
>  	switch (ctx.hw_class) {
>  	case HW_CLASS_R600:
>  	case HW_CLASS_R700:
> +		// If any non-WQM push is invoked, 2 elements should be reserved.
>  		if (has_non_wqm_push)
>  			stack_elements += 2;
>  		break;
>  	case HW_CLASS_CAYMAN:
> +		// If any stack operation is invoked, 2 elements should be reserved
>  		if (stack_elements)
>  			stack_elements += 2;
>  		break;
>  	case HW_CLASS_EVERGREEN:
> -		if (has_non_wqm_push_with_loops_on_stack)
> +		// According to the docs we need to reserve 1 element for each of the
> +		// following cases:
> +		//   1) non-WQM push is used with WQM/LOOP frames on stack
> +		//   2) ALU_ELSE_AFTER is used at the point of max stack usage
> +		// NOTE:
> +		// It was found that the conditions above are not sufficient, there are
> +		// other cases where we also need to reserve stack space, that's why
> +		// we always reserve 1 stack element if we have non-WQM push on stack.
> +		// Condition 2 is ignored for now because we don't use this instruction.
> +		if (has_non_wqm_push)
>  			++stack_elements;

The kernel analyzer reports a stack size of 2 for compute shaders that
have 3 levels of ALU_PUSH_BEFORE.  This would suggest that you either need to
reserve 2 sub-entries (stack_elements in the sb code) when there is a
non-wqm push, or apply the CAYMAN rules to EVERGREEN.

It is possible, though, that the kernel analyzer is over-allocating and
this patch is correct, but I don't have any evidence for this yet.

-Tom


>  		break;
>  	}
> -- 
> 1.8.4.2
> 
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