[mlir][scf] Add no_inline attribute to scf.execute_region (#151352)

Enabling users to explicitly specify which regions should be preserved,
uncovers additional opportunities to utilize `scf.execute_region` op.

Author: jungpark-mlir

mlir/include/mlir/Dialect/SCF/IR/SCFOps.td

@@ -87,6 +87,9 @@ def ExecuteRegionOp : SCF_Op<"execute_region", [
     be accessed inside the op. The op's region can have multiple blocks and the
     blocks can have multiple distinct terminators. Values returned from this op's
     region define the op's results.
+    The optional 'no_inline' flag can be set to request the ExecuteRegionOp to be
+    preserved as much as possible and not being inlined in the parent block until
+    an explicit lowering step.
 
     Example:
 
@@ -98,6 +101,14 @@ def ExecuteRegionOp : SCF_Op<"execute_region", [
       }
     }
 
+    // the same as above but with no_inline attribute
+    scf.for %i = 0 to 128 step %c1 {
+      %y = scf.execute_region -> i32 no_inline {
+        %x = load %A[%i] : memref<128xi32>
+        scf.yield %x : i32
+      }
+    }
+
     affine.for %i = 0 to 100 {
       "foo"() : () -> ()
       %v = scf.execute_region -> i64 {
@@ -119,6 +130,10 @@ def ExecuteRegionOp : SCF_Op<"execute_region", [
     ```
   }];
 
+  let arguments = (ins
+    UnitAttr:$no_inline
+  );
+
   let results = (outs Variadic<AnyType>);
 
   let regions = (region AnyRegion:$region);

mlir/lib/Dialect/SCF/IR/SCF.cpp

@@ -137,6 +137,9 @@ ParseResult ExecuteRegionOp::parse(OpAsmParser &parser,
   if (parser.parseOptionalArrowTypeList(result.types))
     return failure();
 
+  if (succeeded(parser.parseOptionalKeyword("no_inline")))
+    result.addAttribute("no_inline", parser.getBuilder().getUnitAttr());
+
   // Introduce the body region and parse it.
   Region *body = result.addRegion();
   if (parser.parseRegion(*body, /*arguments=*/{}, /*argTypes=*/{}) ||
@@ -148,8 +151,9 @@ ParseResult ExecuteRegionOp::parse(OpAsmParser &parser,
 
 void ExecuteRegionOp::print(OpAsmPrinter &p) {
   p.printOptionalArrowTypeList(getResultTypes());
-
   p << ' ';
+  if (getNoInline())
+    p << "no_inline ";
   p.printRegion(getRegion(),
                 /*printEntryBlockArgs=*/false,
                 /*printBlockTerminators=*/true);
@@ -184,7 +188,7 @@ struct SingleBlockExecuteInliner : public OpRewritePattern<ExecuteRegionOp> {
 
   LogicalResult matchAndRewrite(ExecuteRegionOp op,
                                 PatternRewriter &rewriter) const override {
-    if (!op.getRegion().hasOneBlock())
+    if (!op.getRegion().hasOneBlock() || op.getNoInline())
       return failure();
     replaceOpWithRegion(rewriter, op, op.getRegion());
     return success();

mlir/test/Dialect/SCF/canonicalize.mlir

@@ -1440,8 +1440,8 @@ func.func @propagate_into_execute_region() {
 
 // -----
 
-// CHECK-LABEL: func @execute_region_elim
-func.func @execute_region_elim() {
+// CHECK-LABEL: func @execute_region_inline
+func.func @execute_region_inline() {
   affine.for %i = 0 to 100 {
     "test.foo"() : () -> ()
     %v = scf.execute_region -> i64 {
@@ -1461,8 +1461,30 @@ func.func @execute_region_elim() {
 
 // -----
 
-// CHECK-LABEL: func @func_execute_region_elim
-func.func @func_execute_region_elim() {
+// CHECK-LABEL: func @execute_region_no_inline
+func.func @execute_region_no_inline() {
+  affine.for %i = 0 to 100 {
+    "test.foo"() : () -> ()
+    %v = scf.execute_region -> i64 no_inline {
+      %x = "test.val"() : () -> i64
+      scf.yield %x : i64
+    }
+    "test.bar"(%v) : (i64) -> ()
+  }
+  return
+}
+
+// CHECK-NEXT:     affine.for %arg0 = 0 to 100 {
+// CHECK-NEXT:       "test.foo"() : () -> ()
+// CHECK-NEXT:       scf.execute_region
+// CHECK-NEXT:       %[[VAL:.*]] = "test.val"() : () -> i64
+// CHECK-NEXT:       scf.yield %[[VAL]] : i64
+// CHECK-NEXT:     }
+
+// -----
+
+// CHECK-LABEL: func @func_execute_region_inline
+func.func @func_execute_region_inline() {
     "test.foo"() : () -> ()
     %v = scf.execute_region -> i64 {
       %c = "test.cmp"() : () -> i1
@@ -1496,8 +1518,8 @@ func.func @func_execute_region_elim() {
 
 // -----
 
-// CHECK-LABEL: func @func_execute_region_elim_multi_yield
-func.func @func_execute_region_elim_multi_yield() {
+// CHECK-LABEL: func @func_execute_region_inline_multi_yield
+func.func @func_execute_region_inline_multi_yield() {
     "test.foo"() : () -> ()
     %v = scf.execute_region -> i64 {
       %c = "test.cmp"() : () -> i1