[RS4GC] Fix crash in the case that a live variable has a constant base.

Summary:
Previously, RS4GC crashed in CreateGCRelocates() because it assumed
that every base is also in the array of live variables, which isn't true if a
live variable has a constant base.

This change fixes the crash by making sure CreateGCRelocates() won't try to
relocate a live variable with a constant base.  This would be unnecessary
anyway because anything with a constant base won't move.

Reviewers: reames

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D15556

llvm-svn: 256252

llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

@@ -2387,6 +2387,19 @@ static bool insertParsePoints(Function &F, DominatorTree &DT,
     }
   }
 
+  // It is possible that non-constant live variables have a constant base.  For
+  // example, a GEP with a variable offset from a global.  In this case we can
+  // remove it from the liveset.  We already don't add constants to the liveset
+  // because we assume they won't move at runtime and the GC doesn't need to be
+  // informed about them.  The same reasoning applies if the base is constant.
+  // Note that the relocation placement code relies on this filtering for
+  // correctness as it expects the base to be in the liveset, which isn't true
+  // if the base is constant.
+  for (auto &Info : Records)
+    for (auto &BasePair : Info.PointerToBase)
+      if (isa<Constant>(BasePair.second))
+        Info.LiveSet.erase(BasePair.first);
+
   for (CallInst *CI : Holders)
     CI->eraseFromParent();
 

llvm/test/Transforms/RewriteStatepointsForGC/base-pointers-12.ll

@@ -0,0 +1,20 @@
+; RUN: opt %s -rewrite-statepoints-for-gc -spp-print-base-pointers -S 2>&1 | FileCheck %s
+
+; CHECK: derived %select base %global
+
+@global = external addrspace(1) global i8
+
+define i8 @test(i1 %cond) gc "statepoint-example" {
+  %derived1 = getelementptr i8, i8 addrspace(1)* @global, i64 1
+  %derived2 = getelementptr i8, i8 addrspace(1)* @global, i64 2
+  %select = select i1 %cond, i8 addrspace(1)* %derived1, i8 addrspace(1)* %derived2
+  %safepoint_token = call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @extern, i32 0, i32 0, i32 0, i32 0)
+; CHECK-NOT: relocate
+; CHECK: %load = load i8, i8 addrspace(1)* %select
+  %load = load i8, i8 addrspace(1)* %select
+  ret i8 %load
+}
+
+declare void @extern() gc "statepoint-example"
+
+declare i32 @llvm.experimental.gc.statepoint.p0f_isVoidf(i64, i32, void ()*, i32, i32, ...)

llvm/test/Transforms/RewriteStatepointsForGC/base-pointers-13.ll

@@ -0,0 +1,19 @@
+; RUN: opt %s -rewrite-statepoints-for-gc -spp-print-base-pointers -S 2>&1 | FileCheck %s
+
+; CHECK: derived %derived base %global
+
+@global = external addrspace(1) global i8
+
+define i8 @test(i64 %offset) gc "statepoint-example" {
+  %derived = getelementptr i8, i8 addrspace(1)* @global, i64 %offset
+  %safepoint_token = call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @extern, i32 0, i32 0, i32 0, i32 0)
+; CHECK-NOT: relocate
+; CHECK-NOT: remat
+; CHECK: %load = load i8, i8 addrspace(1)* %derived
+  %load = load i8, i8 addrspace(1)* %derived
+  ret i8 %load
+}
+
+declare void @extern() gc "statepoint-example"
+
+declare i32 @llvm.experimental.gc.statepoint.p0f_isVoidf(i64, i32, void ()*, i32, i32, ...)