[RS4GC] Look through vector bitcasts when looking for base pointer

[dneilson]

Summary

In RS4GC it is possible that a base pointer is contained in a vector that
has undergone a bitcast from one element-pointertype to another. We teach
RS4GC how to look through bitcasts of vector types when looking for a base
pointer.

[anna]

test/Transforms/RewriteStatepointsForGC/vector-bitcast.ll

20

Could you simplify this to a conditional branch and avoid the use of undef here and below in the call?
Can this be triggered just through straightline code?

[dneilson]

Simplify test case

[anna]

LGTM.

lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

if (isa<ShuffleVectorInst>(I))
     // for particular sufflevector patterns.
     return BaseDefiningValueResult(I, false);
 
   // The behavior of getelementptr instructions is the same for vector and
   // non-vector data types.
   if (auto *GEP = dyn_cast<GetElementPtrInst>(I))
     return findBaseDefiningValue(GEP->getPointerOperand());
 
+  // If the pointer comes through a bitcast of a vector of pointers to
+  // a vector of another type of pointer, then look through the bitcast
+  if (auto *BC = dyn_cast<BitCastInst>(I))
+    return findBaseDefiningValue(BC->getOperand(0));
+
   // A PHI or Select is a base defining value.  The outer findBasePointer
   // algorithm is responsible for constructing a base value for this BDV.
   assert((isa<SelectInst>(I) || isa<PHINode>(I)) &&
          "unknown vector instruction - no base found for vector element");
   return BaseDefiningValueResult(I, false);
 }
 
 /// Helper function for findBasePointer - Will return a value which either a)

test/Transforms/RewriteStatepointsForGC/vector-bitcast.ll

+; RUN: opt -S -rewrite-statepoints-for-gc < %s | FileCheck %s
+;
+; A test to make sure that we can look through bitcasts of
+; vector types when a base pointer is contained in a vector.
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128-ni:1"
+target triple = "x86_64-unknown-linux-gnu"
+
+; Function Attrs: uwtable
+define void @test() gc "statepoint-example" {
+; CHECK-LABEL: @test
+entry:
+; CHECK-LABEL: entry
+; CHECK: %bc = bitcast
+; CHECK: %[[p1:[A-Za-z0-9_]+]] = extractelement
+; CHECK: %[[p2:[A-Za-z0-9_]+]] = extractelement
+; CHECK: llvm.experimental.gc.statepoint
+; CHECK: %[[p2]].relocated = {{.+}} @llvm.experimental.gc.relocate
+; CHECK: %[[p1]].relocated = {{.+}} @llvm.experimental.gc.relocate
+; CHECK: load atomic
+  %bc = bitcast <8 x i8 addrspace(1)*> undef to <8 x i32 addrspace(1)*>
+  %ptr= extractelement <8 x i32 addrspace(1)*> %bc, i32 7
+  %0 = call i8 addrspace(1)* undef() [ "deopt"() ]
+  %1 = load atomic i32, i32 addrspace(1)* %ptr unordered, align 4
+  unreachable
+}