Semantic tests for memory invalidation at statepoints

[reames]

Summary

These are simply a collection of tests intended to show that information about the contents of gc references in the heap is lost at a statepoint. I've tried to write them so that they don't disallow correct transformations, while still being fairly easy to understand.

I'd appreciate a sanity check to make sure the tests actually agree with the documentation.

p.s. Ideas for additional tests are welcome.

[sanjoy]

LGTM with the comment corrected.

test/CodeGen/X86/statepoint-forward.ll

67 ↗

(On Diff #16840)

This comment is misleading -- AFAICT what we're checking here is that we can have pointers to GC-ed objects on non GC heaps.

llvm/trunk/test/CodeGen/X86/statepoint-forward.ll

+; RUN: opt -O3 -S < %s | FileCheck --check-prefix=CHECK-OPT %s
+; RUN: llc < %s | FileCheck --check-prefix=CHECK-LLC %s
+; These tests are targetted at making sure we don't retain information
+; about memory which contains potential gc references across a statepoint.
+; They're carefully written to only outlaw forwarding of references. 
+; Depending on the collector, forwarding non-reference fields or
+; constant null references may be perfectly legal. (If unimplemented.)
+; The general structure of these tests is:
+; - learn a fact about memory (via an assume)
+; - cross a statepoint
+; - check the same fact about memory (which we no longer know)
+
+target datalayout = "e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+; If not at a statepoint, we could forward known memory values
+; across this call.
+declare void @func() readonly
+
+;; Forwarding the value of a pointer load is invalid since it may have
+;; changed at the safepoint.  Forwarding a non-gc pointer value would 
+;; be valid, but is not currently implemented.
+define i1 @test_load_forward(i32 addrspace(1)* addrspace(1)* %p) {
+entry:
+  %before = load i32 addrspace(1)* addrspace(1)* %p
+  %cmp1 = call i1 @f(i32 addrspace(1)* %before)
+  call void @llvm.assume(i1 %cmp1)
+  %safepoint_token = tail call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @func, i32 0, i32 0, i32 0, i32 addrspace(1)* addrspace(1)* %p)
+  %pnew = call i32 addrspace(1)* addrspace(1)* @llvm.experimental.gc.relocate.p1p1i32(i32 %safepoint_token, i32 4, i32 4)
+  %after = load i32 addrspace(1)* addrspace(1)* %pnew
+  %cmp2 = call i1 @f(i32 addrspace(1)* %after)
+  ret i1 %cmp2
+
+; CHECK-OPT-LABEL: test_load_forward
+; CHECK-OPT: ret i1 %cmp2
+; CHECK-LLC-LABEL: test_load_forward
+; CHECK-LLC: callq f
+}
+
+;; Same as above, but forwarding from a store
+define i1 @test_store_forward(i32 addrspace(1)* addrspace(1)* %p,
+                              i32 addrspace(1)* %v) {
+entry:
+  %cmp1 = call i1 @f(i32 addrspace(1)* %v)
+  call void @llvm.assume(i1 %cmp1)
+  store i32 addrspace(1)* %v, i32 addrspace(1)* addrspace(1)* %p
+  %safepoint_token = tail call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @func, i32 0, i32 0, i32 0, i32 addrspace(1)* addrspace(1)* %p)
+  %pnew = call i32 addrspace(1)* addrspace(1)* @llvm.experimental.gc.relocate.p1p1i32(i32 %safepoint_token, i32 4, i32 4)
+  %after = load i32 addrspace(1)* addrspace(1)* %pnew
+  %cmp2 = call i1 @f(i32 addrspace(1)* %after)
+  ret i1 %cmp2
+
+; CHECK-OPT-LABEL: test_store_forward
+; CHECK-OPT: ret i1 %cmp2
+; CHECK-LLC-LABEL: test_store_forward
+; CHECK-LLC: callq f
+}
+
+; A predicate on the pointer which is not simply null, but whose value
+; would be known unchanged if the pointer value could be forwarded.
+; The implementation of such a function could inspect the integral value
+; of the pointer and is thus not safe to reuse after a statepoint.
+declare i1 @f(i32 addrspace(1)* %v) readnone
+
+; This is a variant of the test_load_forward test which is intended to 
+; highlight the fact that a gc pointer can be stored in part of the heap
+; that is not itself GC managed.  The GC may have an external mechanism
+; to know about and update that value at a safepoint.  Note that the 
+; statepoint does not provide the collector with this root.
+define i1 @test_load_forward_nongc_heap(i32 addrspace(1)** %p) {
+entry:
+  %before = load i32 addrspace(1)** %p
+  %cmp1 = call i1 @f(i32 addrspace(1)* %before)
+  call void @llvm.assume(i1 %cmp1)
+  call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @func, i32 0, i32 0, i32 0)
+  %after = load i32 addrspace(1)** %p
+  %cmp2 = call i1 @f(i32 addrspace(1)* %after)
+  ret i1 %cmp2
+
+; CHECK-OPT-LABEL: test_load_forward_nongc_heap
+; CHECK-OPT: ret i1 %cmp2
+; CHECK-LLC-LABEL: test_load_forward_nongc_heap
+; CHECK-LLC: callq f
+}
+
+;; Same as above, but forwarding from a store
+define i1 @test_store_forward_nongc_heap(i32 addrspace(1)** %p,
+                                         i32 addrspace(1)* %v) {
+entry:
+  %cmp1 = call i1 @f(i32 addrspace(1)* %v)
+  call void @llvm.assume(i1 %cmp1)
+  store i32 addrspace(1)* %v, i32 addrspace(1)** %p
+  call i32 (void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()* @func, i32 0, i32 0, i32 0)
+  %after = load i32 addrspace(1)** %p
+  %cmp2 = call i1 @f(i32 addrspace(1)* %after)
+  ret i1 %cmp2
+
+; CHECK-OPT-LABEL: test_store_forward_nongc_heap
+; CHECK-OPT: ret i1 %cmp2
+; CHECK-LLC-LABEL: test_store_forward_nongc_heap
+; CHECK-LLC: callq f
+}
+
+
+declare void @llvm.assume(i1)
+declare i32 @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()*, i32, i32, ...)
+declare i32 addrspace(1)* addrspace(1)* @llvm.experimental.gc.relocate.p1p1i32(i32, i32, i32) #3
+