[Mesa-dev] [PATCH 0/5] glsl: Add switch statement support to the GLSL compiler.

[Dan McCabe]

This patch set adds support for switch statements to the GLSL compiler. We
modify the grammar for the compiler with productions for switch statements
and case labels, while adding supporting supporting productions not already 
present. New AST classes are defined to support those productions. However, 
with our approach no new IR is needed, allowing us to leverage all existing 
optimizations and code generation.

Regarding the grammar, we note that the grammar as summarized in the appendix 
of the GLSL specs leaves a bit to be desired. For example, it appears that case 
labels can be used anywhere a statement is valid. However, we note that the
description of the switch statement in Section 6.2 in the body of the spec is 
much more specific, and we follow that text to guide our creation of new
productions.

Specifically, we add productions for:
   switch body,
   case label list,
   case statement, and
   case statement list.
The switch body and the case statement allow us to limit where case labels may
be used.

In turn, we create new AST classes for each of these productions.

For code generation, we generate previously existing IR. Switch statements 
can be thought of a series of if/then/else statements. Case labels are 
compared with the value of a test expression and the case statements are 
executed if the comparison is true.

There are a couple of aspects of switch statements that complicate this 
simplistic view. The primary one is that cases can fall through sequentially
to subsequent cases, unless a break statement is encountered, in which case
the switch statement exits completely.

But break handling is further complicated by the fact that a break statement
can impact the exit of a loop. Thus, we need to coordinate break processing
between switch statements and loop statements.

The code generated by a switch statement maintains three temporary state
variables:
    int test_value;
    bool is_fallthru;
    bool is_break;

test_value is initialized to the value of the test expression at the head of
the switch statement. This is the value that case labels are compared against.

is_fallthru is used to sequentially fall through to subsequent cases and is
initialized to false. When a case label matches the test expression, this
state variable is set to true. It will also be forced to false if a break
statement has been encountered. This forcing to false on break MUST be
after every case test. In practice, we defer that forcing to immediately after
the last case comparison prior to executing a case statement, but that is
an optimization.

is_break is used to indicate that a break statement has been executed and is
initialized to false. When a break statement is encountered, it is set to true.
This state variable is then used to conditionally force is_fallthru to to false
to prevent subsequent case statements from executing.

Code generation for break statements depends on whether the break statement is
inside a switch statement or inside a loop statement. If it inside a loop
statement is inside a break statement, the same code as before gets generated.
But if a switch statement is inside a loop statement, code is emitted to set
the is_break state to true.

Just as ASTs for loop statements are managed in a stack-like manner to handle 
nesting, we also add a bool to capture the innermost switch or loop condition. 
Note that we still need to maintain a loop AST stack to properly handle 
for-loop code generation on a continue statement. Technically, we don't (yet) 
need a switch AST stack, but we are using one for orthogonality with loop 
statements and in anticipation of potential future use. Note that a simple 
boolean stack would have sufficed.

We will illustrate a switch statement with its analogous conditional code that
a switch statement corresponds to by considering an example.

Consider the following switch statement:
	switch (42) {
	case 0:
	case 1:
		gl_FragColor = vec4(1.0, 2.0, 3.0, 4.0);
	case 2:
	case 3:
		gl_FragColor = vec4(4.0, 3.0, 2.0, 1.0);
		break;
	case 4:
	default:
		gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);
	}

Note that case 0 and case 1 fall through to cases 2 and 3 if they occur.

Note that case 4 and the default case must be reached explicitly, since cases
2 and 3 break at the end of their case.

Finally, note that case 4 and the default case don't break but simply fall
through to the end of the switch.

For this code, the equivalent code can be expressed as:
	int test_val = 42; // capture value of test expression
	bool is_fallthru = false; // prevent initial fall throughs
	bool is_break = false; // capture the execution of a break stmt

	is_fallthru |= (test_val == 0); // enable fallthru on case 0
	is_fallthru |= (test_val == 1); // enable fallthru on case 1
	is_fallthru &= !is_break; // inhibit fallthru on previous break
	if (is_fallthru) {
		gl_FragColor = vec4(1.0, 2.0, 3.0, 4.0);
	}

	is_fallthru |= (test_val == 2); // enable fallthru on case 2
	is_fallthru |= (test_val == 3); // enable fallthru on case 3
	is_fallthru &= !is_break; // inhibit fallthru on previous break
	if (is_fallthru) {
		gl_FragColor = vec4(4.0, 3.0, 2.0, 1.0);
		is_break = true; // inhibit all subsequent fallthru for break
	}

	is_fallthru |= (test_val == 4); // enable fallthru on case 4
	is_fallthru = true; // enable fallthru for default case
	is_fallthru &= !is_break; // inhibit fallthru on previous break
	if (is_fallthru) {
		gl_FragColor = vec4(0.0, 0.0, 0.0, 0.0);
	}

The code generate for |= and &= uses the conditional assignment capabilities
of the IR.