[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

1 files changed, 86 insertions, 0 deletions

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]]
+}