[FaultMaps] Let the frontend pre-select implicit null check candidates.

Summary:
This change introduces a !make.implicit metadata that allows the
frontend to pre-select the set of explicit null checks that will be
considered for transformation into implicit null checks.

The reason for not using profiling data instead of !make.implicit is
explained in the change to `FaultMaps.rst`.

Reviewers: atrick, reames, pgavlin, JosephTremoulet

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10824

llvm-svn: 241116

1 files changed, 18 insertions, 2 deletions

diff --git a/llvm/test/CodeGen/X86/implicit-null-check-negative.ll b/llvm/test/CodeGen/X86/implicit-null-check-negative.ll
index 02c3d6e57d1..8fbed9f7bee 100644
--- a/llvm/test/CodeGen/X86/implicit-null-check-negative.ll
+++ b/llvm/test/CodeGen/X86/implicit-null-check-negative.ll
@@ -10,7 +10,7 @@ define i32 @imp_null_check_load(i32* %x, i32* %y) {
   %c = icmp eq i32* %x, null
 ; It isn't legal to move the load from %x from "not_null" to here --
 ; the store to %y could be aliasing it.
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -24,7 +24,7 @@ define i32 @imp_null_check_load(i32* %x, i32* %y) {
 define i32 @imp_null_check_gep_load(i32* %x) {
  entry:
   %c = icmp eq i32* %x, null
-  br i1 %c, label %is_null, label %not_null
+  br i1 %c, label %is_null, label %not_null, !make.implicit !0
 
  is_null:
   ret i32 42
@@ -36,3 +36,19 @@ define i32 @imp_null_check_gep_load(i32* %x) {
   %t = load i32, i32* %x.gep
   ret i32 %t
 }
+
+define i32 @imp_null_check_load_no_md(i32* %x) {
+; This is fine, except it is missing the !make.implicit metadata.
+ entry:
+  %c = icmp eq i32* %x, null
+  br i1 %c, label %is_null, label %not_null
+
+ is_null:
+  ret i32 42
+
+ not_null:
+  %t = load i32, i32* %x
+  ret i32 %t
+}
+
+!0 = !{}