[llvm] r256079 - [RS4GC] Remove an overly strong assertion

[Philip Reames via llvm-commits]

Author: reames
Date: Fri Dec 18 20:38:22 2015
New Revision: 256079

URL: http://llvm.org/viewvc/llvm-project?rev=256079&view=rev
Log:
[RS4GC] Remove an overly strong assertion

As shown by the included test case, it's reasonable to end up with constant references during base pointer calculation.  The code actually handled this case just fine, we only had the assert to help isolate problems under the belief that constant references shouldn't be present in IR generated by managed frontends. This turned out to be wrong on two fronts: 1) Manual Jacobs is working on a language with constant references, and b) we found a case where the optimizer does create them in practice.


Modified:
    llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
    llvm/trunk/test/Transforms/RewriteStatepointsForGC/constants.ll

Modified: llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp?rev=256079&r1=256078&r2=256079&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp Fri Dec 18 20:38:22 2015
@@ -444,16 +444,11 @@ static BaseDefiningValueResult findBaseD
   if (isa<Constant>(I)) {
     assert(!isa<GlobalVariable>(I) && !isa<UndefValue>(I) &&
            "order of checks wrong!");
-    // Note: Finding a constant base for something marked for relocation
-    // doesn't really make sense.  The most likely case is either a) some
-    // screwed up the address space usage or b) your validating against
-    // compiled C++ code w/o the proper separation.  The only real exception
-    // is a null pointer.  You could have generic code written to index of
-    // off a potentially null value and have proven it null.  We also use
-    // null pointers in dead paths of relocation phis (which we might later
-    // want to find a base pointer for).
-    assert(isa<ConstantPointerNull>(I) &&
-           "null is the only case which makes sense");
+    // Note: Even for frontends which don't have constant references, we can
+    // see constants appearing after optimizations.  A simple example is
+    // specialization of an address computation on null feeding into a merge
+    // point where the actual use of the now-constant input is protected by
+    // another null check.  (e.g. test4 in constants.ll)
     return BaseDefiningValueResult(I, true);
   }
 

Modified: llvm/trunk/test/Transforms/RewriteStatepointsForGC/constants.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/RewriteStatepointsForGC/constants.ll?rev=256079&r1=256078&r2=256079&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/RewriteStatepointsForGC/constants.ll (original)
+++ llvm/trunk/test/Transforms/RewriteStatepointsForGC/constants.ll Fri Dec 18 20:38:22 2015
@@ -57,4 +57,39 @@ entry:
   ret i8 %res
 }
 
+; Even for source languages without constant references, we can
+; see constants can show up along paths where the value is dead.
+; This is particular relevant when computing bases of PHIs.  
+define i8 addrspace(1)* @test4(i8 addrspace(1)* %p) gc "statepoint-example" {
+; CHECK-LABEL: @test4
+entry:
+  %is_null = icmp eq i8 addrspace(1)* %p, null
+  br i1 %is_null, label %split, label %join
+
+split:
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0)
+  %arg_value_addr.i = getelementptr inbounds i8, i8 addrspace(1)* %p, i64 8
+  %arg_value_addr_casted.i = bitcast i8 addrspace(1)* %arg_value_addr.i to i8 addrspace(1)* addrspace(1)*
+  br label %join
+
+join:
+; CHECK-LABEL: join
+; CHECK: %addr2.base =
+  %addr2 = phi i8 addrspace(1)* addrspace(1)* [ %arg_value_addr_casted.i, %split ], [ inttoptr (i64 8 to i8 addrspace(1)* addrspace(1)*), %entry ]
+  ;; NOTE: This particular example can be jump-threaded, but in general,
+  ;; we can't, and have to deal with the resulting IR.
+  br i1 %is_null, label %early-exit, label %use
+
+early-exit:
+  ret i8 addrspace(1)* null
+
+use:
+; CHECK-LABEL: use:
+; CHECK: gc.statepoint
+; CHECK: gc.relocate
+  call i32 (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0)
+  %res = load i8 addrspace(1)*, i8 addrspace(1)* addrspace(1)* %addr2, align 1
+  ret i8 addrspace(1)* %res
+}
+