[SimplifyCFG] Fold `llvm.guard(false)` to unreachable

Summary:
`llvm.guard(false)` always bails out of the current compilation unit, so
we can prune any control flow following it.

Reviewers: hfinkel, pcc, reames

Subscribers: majnemer, reames, mcrosier, llvm-commits

Differential Revision: http://reviews.llvm.org/D19245

llvm-svn: 266955

2 files changed, 105 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index bd8b7c45d1a..75eda97d0a3 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -1310,7 +1310,7 @@ static bool markAliveBlocks(Function &F,
       // Assumptions that are known to be false are equivalent to unreachable.
       // Also, if the condition is undefined, then we make the choice most
       // beneficial to the optimizer, and choose that to also be unreachable.
-      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI))
+      if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(BBI)) {
         if (II->getIntrinsicID() == Intrinsic::assume) {
           bool MakeUnreachable = false;
           if (isa<UndefValue>(II->getArgOperand(0)))
@@ -1327,6 +1327,24 @@ static bool markAliveBlocks(Function &F,
           }
         }
 
+        if (II->getIntrinsicID() == Intrinsic::experimental_guard) {
+          // A call to the guard intrinsic bails out of the current compilation
+          // unit if the predicate passed to it is false.  If the predicate is a
+          // constant false, then we know the guard will bail out of the current
+          // compile unconditionally, so all code following it is dead.
+          //
+          // Note: unlike in llvm.assume, it is not "obviously profitable" for
+          // guards to treat `undef` as `false` since a guard on `undef` can
+          // still be useful for widening.
+          if (auto *CI = dyn_cast<ConstantInt>(II->getArgOperand(0)))
+            if (CI->isZero() && !isa<UnreachableInst>(II->getNextNode())) {
+              changeToUnreachable(II->getNextNode(), /*UseLLVMTrap=*/ false);
+              Changed = true;
+              break;
+            }
+        }
+      }
+
       if (CallInst *CI = dyn_cast<CallInst>(BBI)) {
         if (CI->doesNotReturn()) {
           // If we found a call to a no-return function, insert an unreachable
diff --git a/llvm/test/Transforms/SimplifyCFG/guards.ll b/llvm/test/Transforms/SimplifyCFG/guards.ll
new file mode 100644
index 00000000000..71144d7ac24
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/guards.ll
@@ -0,0 +1,86 @@
+; RUN: opt -S -simplifycfg < %s | FileCheck %s
+
+declare void @llvm.experimental.guard(i1, ...)
+
+define i32 @f_0(i1 %c) {
+; CHECK-LABEL: @f_0(
+; CHECK-NEXT: entry:
+; CHECK-NEXT:  call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+; CHECK-NEXT:  unreachable
+entry:
+  call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+  ret i32 10
+}
+
+define i32 @f_1(i1 %c) {
+; Demonstrate that we (intentionally) do not simplify a guard on undef
+
+; CHECK-LABEL: @f_1(
+; CHECK: ret i32 10
+; CHECK: ret i32 20
+
+entry:
+  br i1 %c, label %true, label %false
+
+true:
+  call void(i1, ...) @llvm.experimental.guard(i1 undef) [ "deopt"() ]
+  ret i32 10
+
+false:
+  ret i32 20
+}
+
+define i32 @f_2(i1 %c, i32* %buf) {
+; CHECK-LABEL: @f_2(
+entry:
+  br i1 %c, label %guard_block, label %merge_block
+
+guard_block:
+  call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+  %val = load i32, i32* %buf
+  br label %merge_block
+
+merge_block:
+  %to.return = phi i32 [ %val, %guard_block ], [ 50, %entry ]
+  ret i32 %to.return
+; CHECK: guard_block:
+; CHECK-NEXT:  call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+; CHECK-NEXT:  unreachable
+
+; CHECK: merge_block:
+; CHECK-NEXT:  ret i32 50
+}
+
+define i32 @f_3(i1* %c, i32* %buf) {
+; CHECK-LABEL: @f_3(
+entry:
+  %c0 = load volatile i1, i1* %c
+  br i1 %c0, label %guard_block, label %merge_block
+
+guard_block:
+  call void(i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+  %val = load i32, i32* %buf
+  %c2 = load volatile i1, i1* %c
+  br i1 %c2, label %left, label %right
+
+merge_block:
+  %c1 = load volatile i1, i1* %c
+  br i1 %c1, label %left, label %right
+
+left:
+  %val.left = phi i32 [ %val, %guard_block ], [ 50, %merge_block ]
+  ret i32 %val.left
+
+right:
+  %val.right = phi i32 [ %val, %guard_block ], [ 100, %merge_block ]
+  ret i32 %val.right
+
+; CHECK: guard_block:
+; CHECK-NEXT:   call void (i1, ...) @llvm.experimental.guard(i1 false) [ "deopt"() ]
+; CHECK-NEXT:  unreachable
+
+; CHECK: merge_block:
+; CHECK-NEXT:  %c1 = load volatile i1, i1* %c
+; CHECK-NEXT:  [[VAL:%[^ ]]] = select i1 %c1, i32 50, i32 100
+; CHECK-NEXT:  ret i32 [[VAL]]
+}