cfg: use Tarjan's SCC algorithm to get "minimal loops" (instead of "m…

…aximal"). (#48)

Quoting from the comment I added to the code:
> This analysis was added to because of two observations wrt
"reconvergence":
> 1. syntactic loops (from some high-level language), when truly
structured
> (i.e. only using `while`/`do`-`while` exit conditions, not `break`
etc.),
> *always* map to "minimal loops" on a CFG, as the only loop exit edge
is
> built-in, and no part of the syntactic "loop body" can be its
successor
> 2. more pragmatically, compiling shader languages to SPIR-V seems to
(almost?)
> always *either* fully preserve syntactic loops (via SPIR-V
`OpLoopMerge`),
> *or* structurize CFGs in a way that produces "minimal loops", which
can
> be misleading with explicit `break`s (moving user code from just
before
> the `break` to after the loop), but is less impactful than "maximal
loops"

See also:
* #11
(of which this only fixes the loop side, and doesn't even keep the
bitflags)

CHANGELOG.md

@@ -33,10 +33,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased] - ReleaseDate
 
+### Changed 🛠
+- [PR#48](https://github.com/EmbarkStudios/spirt/pull/48) changed CFG structurization
+  from "maximal loops" to "minimal loops" (computed using Tarjan's SCC algorithm),
+  and added `OpLoopMerge` support on top (by extending a "minimal loop" as needed)
+
 ## [0.3.0] - 2023-07-25
 
 ### Added ⭐
-- [PR#45](https://github.com/EmbarkStudios/spirt/pull/45) added the abilit to
+- [PR#45](https://github.com/EmbarkStudios/spirt/pull/45) added the ability to
   pretty-print `OpExtInst`s (SPIR-V "extended instructions") using official
   `extinst.*.grammar.json` descriptions and/or custom ones (registered via `Context`)
 

README.md

@@ -140,16 +140,16 @@ global_var GV0 in spv.StorageClass.Output: s32
 func F0() -> spv.OpTypeVoid {
   loop(v0: s32 <- 1s32, v1: s32 <- 1s32) {
     v2 = spv.OpSLessThan(v1, 10s32): bool
-    (v3: bool, v4: s32, v5: s32) = if v2 {
+    (v3: bool, v4: s32, v5: s32, _: bool) = if v2 {
       v6 = spv.OpIMul(v0, v1): s32
       v7 = spv.OpIAdd(v1, 1s32): s32
-      (true, v6, v7)
+      (true, v6, v7, false)
     } else {
-      spv.OpStore(Pointer: &GV0, Object: v0)
-      (false, spv.OpUndef: s32, spv.OpUndef: s32)
+      (false, spv.OpUndef: s32, spv.OpUndef: s32, true)
     }
     (v4, v5) -> (v0, v1)
   } while v3
+  spv.OpStore(Pointer: &GV0, Object: v0)
 }
 ```
 <!-- END tests/data/for-loop.wgsl.spvasm.structured.spirt -->

build.rs

@@ -32,17 +32,17 @@ fn main() {
         .collect();
     extinsts_names_and_grammars.sort();
 
-    let all_jsons = format!(
+    let mut all_jsons = format!(
         "pub(super) const SPIRV_CORE_GRAMMAR: &str = include_str!({:?});\n\
-         pub(super) const EXTINST_NAMES_AND_GRAMMARS: &[(&str, &str)] = &[\n{}];",
+         pub(super) const EXTINST_NAMES_AND_GRAMMARS: &[(&str, &str)] = &[\n",
         khr_spv_include_dir.join(core_grammar),
-        extinsts_names_and_grammars
-            .into_iter()
-            .map(|(name, grammar)| {
-                format!("({:?}, include_str!({:?})),\n", name, khr_spv_include_dir.join(grammar),)
-            })
-            .collect::<String>()
     );
+    for (name, grammar) in extinsts_names_and_grammars {
+        use std::fmt::Write as _;
+        writeln!(all_jsons, "({:?}, include_str!({:?})),", name, khr_spv_include_dir.join(grammar))
+            .unwrap();
+    }
+    all_jsons += "];";
     std::fs::write(
         std::path::PathBuf::from(std::env::var_os("OUT_DIR").unwrap())
             .join("khr_spv_grammar_jsons.rs"),