%ptr = call i32* @get_ptr()
%ptr_is_null = icmp i32* %ptr, null
- br i1 %ptr_is_null, label %is_null, label %not_null
+ br i1 %ptr_is_null, label %is_null, label %not_null, !make.implicit !0
not_null:
%t = load i32, i32* %ptr
is_null:
call void @HFC()
unreachable
+
+ !0 = !{}
to control flow implicit in the instruction loading or storing through
the pointer being null checked:
The ``ImplicitNullChecks`` pass adds entries to the
``__llvm_faultmaps`` section described above as needed.
+
+``make.implicit`` metadata
+--------------------------
+
+Making null checks implicit is an aggressive optimization, and it can
+be a net performance pessimization if too many memory operations end
+up faulting because of it. A language runtime typically needs to
+ensure that only a negligible number of implicit null checks actually
+fault once the application has reached a steady state. A standard way
+of doing this is by healing failed implicit null checks into explicit
+null checks via code patching or recompilation. It follows that there
+are two requirements an explicit null check needs to satisfy for it to
+be profitable to convert it to an implicit null check:
+
+ 1. The case where the pointer is actually null (i.e. the "failing"
+ case) is extremely rare.
+
+ 2. The failing path heals the implicit null check into an explicit
+ null check so that the application does not repeatedly page
+ fault.
+
+The frontend is expected to mark branches that satisfy (1) and (2)
+using a ``!make.implicit`` metadata node (the actual content of the
+metadata node is ignored). Only branches that are marked with
+``!make.implicit`` metadata are considered as candidates for
+conversion into implicit null checks.
+
+(Note that while we could deal with (1) using profiling data, dealing
+with (2) requires some information not present in branch profiles.)
MachineBasicBlock &MBB, SmallVectorImpl<NullCheck> &NullCheckList) {
typedef TargetInstrInfo::MachineBranchPredicate MachineBranchPredicate;
+ MDNode *BranchMD =
+ MBB.getBasicBlock()
+ ? MBB.getBasicBlock()->getTerminator()->getMetadata("make.implicit")
+ : nullptr;
+ if (!BranchMD)
+ return false;
+
MachineBranchPredicate MBP;
if (TII->AnalyzeBranchPredicate(MBB, MBP, true))
%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
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
%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 = !{}
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
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
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
ret i32 %p1
}
+!0 = !{}
+
; CHECK-LABEL: __LLVM_FaultMaps:
; Version: