Fix infinite loop in SPIRVLegalizationContext::processWorkList (#8712)

When slangc is invoked with -g, a source shader that has static infinite
loop can generate IR that have branch to a block that contains a branch
to the first block that contains the first branch, resulting in infinite
loop.

Change SPIRVLegalizationContext::processWorkList to only add branch
target to work list via its parent, this avoids the infinite loop above.

Also change addToWorkList to stop addUsersToWorkList, users should be
added explicitly by logic for specific insts.

Add regression test as tests/spirv/infinite-loop.slang

Fixes #8669

1 files changed, 17 insertions, 0 deletions

diff --git a/tests/spirv/infinite-loop.slang b/tests/spirv/infinite-loop.slang
new file mode 100644
index 000000000..1562f7334
--- /dev/null
+++ b/tests/spirv/infinite-loop.slang
@@ -0,0 +1,17 @@
+// Simple regression test for SPIRV compilation hang when there is cycle between unconditional branch and block.
+// The -g was necessary to reproduce the hang prior to fix for https://github.com/shader-slang/slang/issues/8669
+
+//TEST:SIMPLE(filecheck=CHECK): -target spirv -stage fragment -entry main -g
+//CHECK: main
+//CHECK: OpBranch
+//CHECK: OpUnreachable
+
+layout(location = 0) out float4 output_color;
+[shader("fragment")]
+void main()
+{
+    while (true) {
+        int i = 0;
+    }
+    output_color = float4(1, 2, 3, 4);
+}