[Safepoint Verifier] gc.relocate does not change the constant property.

[skatkov]

Summary

Add traverse through gc.relocate in determining whether base is constant.

[anna]

LGTM with a comment. Great job on root causing and fixing this!

JFYI, this was *one of the false positives* we could see in SafepointVerifier. We just never saw this in practice. It just started showing up a lot more in our downstream code after this NFC: https://reviews.llvm.org/D118604

llvm/lib/IR/SafepointIRVerifier.cpp

360

Very nice. Could you pls add a comment here as well, that GCRelocates do not change null-ness or constant-ness of the value. So, this would allow us to recognize if the value is either: isExclusivelyDerivedFromNull OR ExclusivelyNull.

[reames]

LGTM as well.

llvm/lib/IR/SafepointIRVerifier.cpp

if (const auto *PN = dyn_cast<PHINode>(V)) {
       continue;
     }
     if (const auto *SI = dyn_cast<SelectInst>(V)) {
       // Push in the true and false values
       Worklist.push_back(SI->getTrueValue());
       Worklist.push_back(SI->getFalseValue());
       continue;
     }
+    if (const auto *GCRelocate = dyn_cast<GCRelocateInst>(V)) {
+      Worklist.push_back(GCRelocate->getDerivedPtr());
+      continue;
+    }
     if (isa<Constant>(V)) {
       // We found at least one base pointer which is non-null, so this derived
       // pointer is not exclusively derived from null.
       if (V != Constant::getNullValue(V->getType()))
         isExclusivelyDerivedFromNull = false;
       // Continue processing the remaining values to make sure it's exclusively
       // constant.
       continue;

llvm/test/SafepointIRVerifier/hidden-constant-base.ll

 ; RUN: opt -safepoint-ir-verifier-print-only -verify-safepoint-ir -S %s 2>&1 | FileCheck %s
 
-; This is a false positive case. Actually base of %tmp5 is null but verifier does not see that.
-; As a result it states that %tmp5 can have a unrelocated value of %tmp.
-; Relocation of %tmp is expected to be after the call to @widget but it is not required
-; due to %tmp is always null as well.
-
 ; CHECK-LABEL: Verifying gc pointers in function: test
-; CHECK: Illegal use of unrelocated value found!
+; CHECK: No illegal uses found by SafepointIRVerifier in: test
 define i8 addrspace(1)* @test(i1 %arg) gc "statepoint-example" {
 bb:
   br label %bb2
 
 bb2:                                              ; preds = %bb8, %bb
   %tmp = phi i8 addrspace(1)* [ %tmp5.relocated, %bb8 ], [ null, %bb ]
   %statepoint_token = call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* elementtype(void ()) @widget, i32 0, i32 0, i32 0, i32 0) [ "deopt"() ]
   br label %bb4