[PATCH] D101180: Mark type test intrinsics as speculatable to fix inline cost

[Teresa Johnson via Phabricator via llvm-commits]

tejohnson created this revision.
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There is already code in InlineCost.cpp to identify and ignore ephemeral
values (llvm.assume intrinsics and other side-effect free instructions
only feeding the assumes). However, because llvm.type.test intrinsics
were not marked speculatable, they and any instructions specifically
feeding the type test (typically a bitcast) were being counted towards
the instruction cost when inlining. This was causing profile matching
issues in some cases when enabling -fwhole-program-vtables for whole
program devirtualization.

According to the language reference, the speculatable attribute means:
"the function does not have any effects besides calculating its result
and does not have undefined behavior". I see no reason why type tests
cannot be marked with this attribute.

There are 2 test changes:

llvm/test/Transforms/Inline/ephemeral.ll: I added a type test intrinsic
here to verify the fix. Also, I found the test was not actually testing
what it originally intended. Many of the existing instructions were
optimized away by -Oz, and the cost of inlining was negative due to the
benefit of removing the call. So I changed the test to simply invoke the
inline pass and check the number of instructions computed by InlineCost.
I also fixed an instruction that was not actually used anywhere.

llvm/test/Transforms/SimplifyCFG/no-md-sink.ll needed to be made more
robust to code changes that reordered the metadata.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D101180

Files:
  llvm/include/llvm/IR/Intrinsics.td
  llvm/test/Transforms/Inline/ephemeral.ll
  llvm/test/Transforms/SimplifyCFG/no-md-sink.ll


Index: llvm/test/Transforms/SimplifyCFG/no-md-sink.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/no-md-sink.ll
+++ llvm/test/Transforms/SimplifyCFG/no-md-sink.ll
@@ -47,6 +47,6 @@
 
 ; CHECK-LABEL: test2
 ; CHECK: %[[S:[a-z0-9.]+]] = select i1 %flag, i8* %y, i8* %z
-; CHECK: %[[R:[a-z0-9.]+]] = call i1 @llvm.type.test(i8* %[[S]], metadata !0)
+; CHECK: %[[R:[a-z0-9.]+]] = call i1 @llvm.type.test(i8* %[[S]], metadata ![[MD:[0-9]+]]
 ; CHECK: ret i1 %[[R]]
-
+; CHECK: ![[MD]] = !{i32 0, !"typeid1"}
Index: llvm/test/Transforms/Inline/ephemeral.ll
===================================================================
--- llvm/test/Transforms/Inline/ephemeral.ll
+++ llvm/test/Transforms/Inline/ephemeral.ll
@@ -1,30 +1,35 @@
-; RUN: opt -S -Oz %s | FileCheck %s
+; RUN: opt -S -inline %s -debug-only=inline-cost 2>&1 | FileCheck %s
+; Only the load and ret should be included in the instruction count, not
+; the instructions feeding the assume.
+; CHECK: NumInstructions: 2
 
 @a = global i32 4
 
-define i32 @inner() {
+define i32 @inner(i8* %y) {
   %a1 = load volatile i32, i32* @a
 
-  ; Here are enough instructions to prevent inlining, but because they are used
-  ; only by the @llvm.assume intrinsic, they're free (and, thus, inlining will
-  ; still happen).
+  ; Because these instructions are used only by the @llvm.assume intrinsic,
+  ; they're free and should not be included in the instruction count when
+  ; computing the inline cost.
   %a2 = mul i32 %a1, %a1
-  %a3 = sub i32 %a1, 5
+  %a3 = sub i32 %a1, %a2
   %a4 = udiv i32 %a3, -13
   %a5 = mul i32 %a4, %a4
   %a6 = add i32 %a5, %a5
   %ca = icmp sgt i32 %a6, -7
-  tail call void @llvm.assume(i1 %ca)
+  %r = call i1 @llvm.type.test(i8* %y, metadata !0)
+  %ca2 = icmp eq i1 %ca, %r
+  tail call void @llvm.assume(i1 %ca2)
 
   ret i32 %a1
 }
 
-; @inner() should be inlined for -Oz.
-; CHECK-NOT: call i1 @inner
-define i32 @outer() optsize {
-   %r = call i32 @inner()
+define i32 @outer(i8* %y) optsize {
+   %r = call i32 @inner(i8* %y)
    ret i32 %r
 }
 
 declare void @llvm.assume(i1) nounwind
+declare i1 @llvm.type.test(i8*, metadata) nounwind readnone
 
+!0 = !{i32 0, !"typeid1"}
Index: llvm/include/llvm/IR/Intrinsics.td
===================================================================
--- llvm/include/llvm/IR/Intrinsics.td
+++ llvm/include/llvm/IR/Intrinsics.td
@@ -1461,7 +1461,7 @@
 
 // Test whether a pointer is associated with a type metadata identifier.
 def int_type_test : DefaultAttrsIntrinsic<[llvm_i1_ty], [llvm_ptr_ty, llvm_metadata_ty],
-                              [IntrNoMem, IntrWillReturn]>;
+                              [IntrNoMem, IntrWillReturn, IntrSpeculatable]>;
 
 // Safely loads a function pointer from a virtual table pointer using type metadata.
 def int_type_checked_load : DefaultAttrsIntrinsic<[llvm_ptr_ty, llvm_i1_ty],


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