ir3/ra: Fix off-by-one issues with live-range extension

The intersects() function assumes that inside each instruction values
always die before they are defined, so that if the end of one range is
the same instruction as the beginning of the next then they don't
intersect. However, this isn't the case for values that become live at
the beginning of a basic block, which become live *before* the first
instruction, or instructions that die at the end of a basic block which
die after the last instruction.

For example, imagine that we have two values, A which is defined earlier
in the block and B which is defined in the last instruction of the block
and both die at the end of the basic block (e.g. are used in the next
iteration of a loop). We would compute a range for A of, say, (10, 20)
and for B of (20, 20) since each block's end_ip is the same as the ip of
the last instruction, and RA would consider them to not interfere.
There's a similar problem with values that become live at the beginning.

The fix is to offset the block's start_ip and end_ip by one so that they
don't correspond to any actual instruction. One way to think about this
is that we're adding fake instructions at the beginning and end of a
block where values become live & die. We could invert the order, so that
values consumed by each instruction are considered dead at the end of
the previous instruction, but then values that become dead at the
beginning of the basic block would incorrectly have an empty live range,
with a similar problem at the end of the basic block if we try to say
that values are defined at the beginning of the next instruction. So
the extra padding instructions are unavoidable.

This fixes an accidental infinite loop in the shader for
dEQP-VK.spirv_assembly.type.scalar.u32.switch_vert.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4614>

3 files changed, 34 insertions, 2 deletions

diff --git a/src/freedreno/ir3/ir3.c b/src/freedreno/ir3/ir3.c
index 9678389e8b5..d4fcd995ed0 100644
--- a/src/freedreno/ir3/ir3.c
+++ b/src/freedreno/ir3/ir3.c
@@ -1155,11 +1155,33 @@ ir3_count_instructions(struct ir3 *ir)
 	unsigned cnt = 1;
 	foreach_block (block, &ir->block_list) {
 		block->start_ip = cnt;
+		foreach_instr (instr, &block->instr_list) {
+			instr->ip = cnt++;
+		}
 		block->end_ip = cnt;
+	}
+	return cnt;
+}
+
+/* When counting instructions for RA, we insert extra fake instructions at the
+ * beginning of each block, where values become live, and at the end where
+ * values die. This prevents problems where values live-in at the beginning or
+ * live-out at the end of a block from being treated as if they were
+ * live-in/live-out at the first/last instruction, which would be incorrect.
+ * In ir3_legalize these ip's are assumed to be actual ip's of the final
+ * program, so it would be incorrect to use this everywhere.
+ */
+
+unsigned
+ir3_count_instructions_ra(struct ir3 *ir)
+{
+	unsigned cnt = 1;
+	foreach_block (block, &ir->block_list) {
+		block->start_ip = cnt++;
 		foreach_instr (instr, &block->instr_list) {
 			instr->ip = cnt++;
-			block->end_ip = instr->ip;
 		}
+		block->end_ip = cnt++;
 	}
 	return cnt;
 }
diff --git a/src/freedreno/ir3/ir3.h b/src/freedreno/ir3/ir3.h
index 351490aecf7..6cb882002f0 100644
--- a/src/freedreno/ir3/ir3.h
+++ b/src/freedreno/ir3/ir3.h
@@ -594,6 +594,7 @@ void ir3_block_clear_mark(struct ir3_block *block);
 void ir3_clear_mark(struct ir3 *shader);
 
 unsigned ir3_count_instructions(struct ir3 *ir);
+unsigned ir3_count_instructions_ra(struct ir3 *ir);
 
 void ir3_find_ssa_uses(struct ir3 *ir, void *mem_ctx, bool falsedeps);
 
diff --git a/src/freedreno/ir3/ir3_ra.c b/src/freedreno/ir3/ir3_ra.c
index 72682dc6988..f74174498d0 100644
--- a/src/freedreno/ir3/ir3_ra.c
+++ b/src/freedreno/ir3/ir3_ra.c
@@ -541,7 +541,7 @@ ra_init(struct ir3_ra_ctx *ctx)
 	unsigned n, base;
 
 	ir3_clear_mark(ctx->ir);
-	n = ir3_count_instructions(ctx->ir);
+	n = ir3_count_instructions_ra(ctx->ir);
 
 	ctx->instrd = rzalloc_array(NULL, struct ir3_ra_instr_data, n);
 
@@ -950,6 +950,15 @@ ra_calc_block_live_values(struct ir3_ra_ctx *ctx, struct ir3_block *block)
 
 	/* the remaining live should match liveout (for extra sanity testing): */
 	if (RA_DEBUG) {
+		unsigned new_dead = 0;
+		BITSET_FOREACH_SET (name, live, ctx->alloc_count) {
+			/* Is this the last use? */
+			if (ctx->use[name] != block->end_ip)
+				continue;
+			new_dead += name_size(ctx, name);
+			d("NEW_DEAD: %u (new_dead=%u)", name, new_dead);
+			BITSET_CLEAR(live, name);
+		}
 		unsigned liveout = 0;
 		BITSET_FOREACH_SET (name, bd->liveout, ctx->alloc_count) {
 			liveout += name_size(ctx, name);