[Mesa-dev] [PATCH] r600g: Workaround for a nested loop bug on Cayman

[Martin Andersson]

There is a bug where a BREAK/CONTINUE followed by LOOP_STARTxxx for nested
loops may put the branch stack into a state such that ALU_PUSH_BEFORE
doesn't work as expected. Workaround this by replacing the ALU_PUSH_BEFORE
with a PUSH + ALU for nested loops.

Fixes piglit tests:
spec/!OpenGL 1.1/read-front
spec/EXT_transform_feedback/order*
spec/glsl-1.40/uniform_buffer/fs-struct-pad

No piglit regressions.
---
 src/gallium/drivers/r600/r600_shader.c | 33 ++++++++++++++++++++++++++++++---
 1 file changed, 30 insertions(+), 3 deletions(-)

diff --git a/src/gallium/drivers/r600/r600_shader.c b/src/gallium/drivers/r600/r600_shader.c
index 6dbca50..aee011e 100644
--- a/src/gallium/drivers/r600/r600_shader.c
+++ b/src/gallium/drivers/r600/r600_shader.c
@@ -252,6 +252,7 @@ static int tgsi_endif(struct r600_shader_ctx *ctx);
 static int tgsi_bgnloop(struct r600_shader_ctx *ctx);
 static int tgsi_endloop(struct r600_shader_ctx *ctx);
 static int tgsi_loop_brk_cont(struct r600_shader_ctx *ctx);
+static bool need_cayman_loop_bug_workaround(struct r600_shader_ctx *ctx);
 
 /*
  * bytestream -> r600 shader
@@ -5490,7 +5491,7 @@ static int tgsi_opdst(struct r600_shader_ctx *ctx)
 	return 0;
 }
 
-static int emit_logic_pred(struct r600_shader_ctx *ctx, int opcode)
+static int emit_logic_pred(struct r600_shader_ctx *ctx, int opcode, int alu_type)
 {
 	struct r600_bytecode_alu alu;
 	int r;
@@ -5510,7 +5511,7 @@ static int emit_logic_pred(struct r600_shader_ctx *ctx, int opcode)
 
 	alu.last = 1;
 
-	r = r600_bytecode_add_alu_type(ctx->bc, &alu, CF_OP_ALU_PUSH_BEFORE);
+	r = r600_bytecode_add_alu_type(ctx->bc, &alu, alu_type);
 	if (r)
 		return r;
 	return 0;
@@ -5730,7 +5731,20 @@ static void break_loop_on_flag(struct r600_shader_ctx *ctx, unsigned fc_sp)
 
 static int tgsi_if(struct r600_shader_ctx *ctx)
 {
-	emit_logic_pred(ctx, ALU_OP2_PRED_SETNE_INT);
+	int alu_type = CF_OP_ALU_PUSH_BEFORE;
+
+	/* 
+	   There is a bug where a BREAK/CONTINUE followed by LOOP_STARTxxx for nested
+	   loops may put the branch stack into a state such that ALU_PUSH_BEFORE 
+	   doesn't work as expected. Workaround this by replacing the ALU_PUSH_BEFORE
+	   with a PUSH + ALU for nested loops.
+	 */
+	if (ctx->bc->chip_class == CAYMAN && need_cayman_loop_bug_workaround(ctx)) {
+		r600_bytecode_add_cfinst(ctx->bc, CF_OP_PUSH);
+		alu_type = CF_OP_ALU;
+	}
+
+	emit_logic_pred(ctx, ALU_OP2_PRED_SETNE_INT, alu_type);
 
 	r600_bytecode_add_cfinst(ctx->bc, CF_OP_JUMP);
 
@@ -5834,6 +5848,19 @@ static int tgsi_loop_brk_cont(struct r600_shader_ctx *ctx)
 	return 0;
 }
 
+static bool need_cayman_loop_bug_workaround(struct r600_shader_ctx *ctx)
+{
+	unsigned int fscp;
+	int num_loops = 0;
+	for (fscp = ctx->bc->fc_sp; fscp > 0; fscp--)
+	{
+		if (FC_LOOP == ctx->bc->fc_stack[fscp].type)
+			++num_loops;
+	}
+
+	return num_loops >= 2;
+}
+
 static int tgsi_umad(struct r600_shader_ctx *ctx)
 {
 	struct tgsi_full_instruction *inst = &ctx->parse.FullToken.FullInstruction;
-- 
1.8.2.1