Lower widenable_conditions in CGP

[reames]

Summary

This ensures that if we make it to the backend w/o lowering widenable_conditions first, that we generate correct code. Doing it in CGP - instead of isel - let's us fold control flow before hitting block local instruction selection.

[mkazantsev]

LGTM

lib/CodeGen/CodeGenPrepare.cpp

for (auto &Arg : CI->arg_operands()) {
       unsigned AS = Arg->getType()->getPointerAddressSpace();
       return optimizeMemoryInst(CI, Arg, Arg->getType(), AS);
     }
 
   IntrinsicInst *II = dyn_cast<IntrinsicInst>(CI);
   if (II) {
     switch (II->getIntrinsicID()) {
     default: break;
+    case Intrinsic::experimental_widenable_condition: {
+      // Give up on future widening oppurtunties so that we can fold away dead
+      // paths and merge blocks before going into block-local instruction
+      // selection.   
+      if (II->use_empty()) {
+        II->eraseFromParent();
+        return true;
+      }
+      Constant *RetVal = ConstantInt::getTrue(II->getContext());
+      resetIteratorIfInvalidatedWhileCalling(BB, [&]() {
+        replaceAndRecursivelySimplify(CI, RetVal, TLInfo, nullptr);
+      });
+      return true;
+    }
     case Intrinsic::objectsize: {
       // Lower all uses of llvm.objectsize.*
       ConstantInt *RetVal =
           lowerObjectSizeCall(II, *DL, TLInfo, /*MustSucceed=*/true);
 
       resetIteratorIfInvalidatedWhileCalling(BB, [&]() {
         replaceAndRecursivelySimplify(CI, RetVal, TLInfo, nullptr);
       });

test/Transforms/CodeGenPrepare/widenable-condition.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -codegenprepare -S < %s | FileCheck %s
+
+; Check the idiomatic guard pattern to ensure it's lowered correctly.
+define void @test_guard(i1 %cond_0) {
+; CHECK-LABEL: @test_guard(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND_0:%.*]], label [[GUARDED:%.*]], label [[DEOPT:%.*]]
+; CHECK:       deopt:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    ret void
+; CHECK:       guarded:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %guarded, label %deopt
+
+deopt:                                            ; preds = %entry
+  call void @foo()
+  ret void
+
+guarded:
+  ret void
+}
+
+;; Test a non-guard fastpath/slowpath case
+define void @test_triangle(i1 %cond_0) {
+; CHECK-LABEL: @test_triangle(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br i1 [[COND_0:%.*]], label [[FASTPATH:%.*]], label [[SLOWPATH:%.*]]
+; CHECK:       fastpath:
+; CHECK-NEXT:    call void @bar()
+; CHECK-NEXT:    br label [[MERGE:%.*]]
+; CHECK:       slowpath:
+; CHECK-NEXT:    call void @foo()
+; CHECK-NEXT:    br label [[MERGE]]
+; CHECK:       merge:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %widenable_cond = call i1 @llvm.experimental.widenable.condition()
+  %exiplicit_guard_cond = and i1 %cond_0, %widenable_cond
+  br i1 %exiplicit_guard_cond, label %fastpath, label %slowpath
+
+fastpath:
+  call void @bar()
+  br label %merge
+
+slowpath:
+  call void @foo()
+  br label %merge
+
+merge:
+  ret void
+}
+
+
+; Demonstrate that resulting CFG simplifications are made
+define void @test_cfg_simplify() {
+; CHECK-LABEL: @test_cfg_simplify(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %widenable_cond3 = call i1 @llvm.experimental.widenable.condition()
+  br i1 %widenable_cond3, label %guarded2, label %deopt3
+
+deopt3:
+  call void @foo()
+  ret void
+
+guarded2:
+  %widenable_cond4 = call i1 @llvm.experimental.widenable.condition()
+  br i1 %widenable_cond4, label %merge1, label %slowpath1
+
+slowpath1:
+  call void @foo()
+  br label %merge1
+
+merge1:
+  ret void
+}
+
+
+declare void @foo()
+declare void @bar()
+
+; Function Attrs: inaccessiblememonly nounwind
+declare i1 @llvm.experimental.widenable.condition() #0
+
+attributes #0 = { inaccessiblememonly nounwind }