Change merge return pass to handle structured cfg.

We are seeing shaders that have multiple returns in a functions.  These
functions must get inlined for legalization purposes; however, the
inliner does not know how to inline functions that have multiple
returns.

The solution we will go with it to improve the merge return pass to
handle structured control flow.

Note that the merge return pass will assume the cfg has been cleanedup
by dead branch elimination.

Fixes #857.

1 files changed, 11 insertions, 8 deletions

diff --git a/include/spirv-tools/optimizer.hpp b/include/spirv-tools/optimizer.hpp
index 03eb1c29..d0907ac2 100644
--- a/include/spirv-tools/optimizer.hpp
+++ b/include/spirv-tools/optimizer.hpp
@@ -453,16 +453,19 @@ Optimizer::PassToken CreateCFGCleanupPass();
 // that are not referenced.
 Optimizer::PassToken CreateDeadVariableEliminationPass();
 
-// Create merge return pass.
-// This pass replaces all returns with unconditional branches to a new block
-// containing a return. If necessary, this new block will contain a PHI node to
-// select the correct return value.
+// create merge return pass.
+// changes functions that have multiple return statements so they have a single
+// return statement.
 //
-// This pass does not consider unreachable code, nor does it perform any other
-// optimizations.
+// for structured control flow it is assumed that the only unreachable blocks in
+// the function are trivial merge and continue blocks.
 //
-// This pass does not currently support structured control flow. It bails out if
-// the shader capability is detected.
+// a trivial merge block contains the label and an opunreachable instructions,
+// nothing else.  a trivial continue block contain a label and an opbranch to
+// the header, nothing else.
+//
+// these conditions are guaranteed to be met after running dead-branch
+// elimination.
 Optimizer::PassToken CreateMergeReturnPass();
 
 // Create value numbering pass.