[llvm] r224968 - Carry facts about nullness and undef across GC relocation

[Philip Reames]

Author: reames
Date: Mon Dec 29 17:27:30 2014
New Revision: 224968

URL: http://llvm.org/viewvc/llvm-project?rev=224968&view=rev
Log:
Carry facts about nullness and undef across GC relocation

This change implements four basic optimizations:

    If a relocated value isn't used, it doesn't need to be relocated.
    If the value being relocated is null, relocation doesn't change that. (Technically, this might be collector specific. I don't know of one which it doesn't work for though.)
    If the value being relocated is undef, the relocation is meaningless.
    If the value being relocated was known nonnull, the relocated pointer also isn't null. (Since it points to the same source language object.)

I outlined other planned work in comments.

Differential Revision: http://reviews.llvm.org/D6600


Added:
    llvm/trunk/test/Transforms/InstCombine/statepoint.ll
Modified:
    llvm/trunk/include/llvm/IR/Statepoint.h
    llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp

Modified: llvm/trunk/include/llvm/IR/Statepoint.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/IR/Statepoint.h?rev=224968&r1=224967&r2=224968&view=diff
==============================================================================
--- llvm/trunk/include/llvm/IR/Statepoint.h (original)
+++ llvm/trunk/include/llvm/IR/Statepoint.h Mon Dec 29 17:27:30 2014
@@ -202,11 +202,11 @@ class GCRelocateOperands {
   int derivedPtrIndex() {
     return cast<ConstantInt>(RelocateCS.getArgument(2))->getZExtValue();
   }
-  const Value *basePtr() {
+  Value *basePtr() {
     ImmutableCallSite CS(statepoint());
     return *(CS.arg_begin() + basePtrIndex());
   }
-  const Value *derivedPtr() {
+  Value *derivedPtr() {
     ImmutableCallSite CS(statepoint());
     return *(CS.arg_begin() + derivedPtrIndex());
   }

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp?rev=224968&r1=224967&r2=224968&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCalls.cpp Mon Dec 29 17:27:30 2014
@@ -1134,6 +1134,42 @@ Instruction *InstCombiner::visitCallInst
 
     break;
   }
+  case Intrinsic::experimental_gc_relocate: {
+    // Translate facts known about a pointer before relocating into
+    // facts about the relocate value, while being careful to
+    // preserve relocation semantics.
+    GCRelocateOperands Operands(II);
+    Value *DerivedPtr = Operands.derivedPtr();
+
+    // Remove the relocation if unused, note that this check is required
+    // to prevent the cases below from looping forever.
+    if (II->use_empty())
+      return EraseInstFromFunction(*II);
+
+    // Undef is undef, even after relocation.
+    // TODO: provide a hook for this in GCStrategy.  This is clearly legal for
+    // most practical collectors, but there was discussion in the review thread
+    // about whether it was legal for all possible collectors.
+    if (isa<UndefValue>(DerivedPtr))
+      return ReplaceInstUsesWith(*II, DerivedPtr);
+
+    // The relocation of null will be null for most any collector.
+    // TODO: provide a hook for this in GCStrategy.  There might be some weird
+    // collector this property does not hold for.
+    if (isa<ConstantPointerNull>(DerivedPtr))
+      return ReplaceInstUsesWith(*II, DerivedPtr);
+
+    // isKnownNonNull -> nonnull attribute
+    if (isKnownNonNull(DerivedPtr))
+      II->addAttribute(AttributeSet::ReturnIndex, Attribute::NonNull);
+
+    // TODO: dereferenceable -> deref attribute
+
+    // TODO: bitcast(relocate(p)) -> relocate(bitcast(p))
+    // Canonicalize on the type from the uses to the defs
+    
+    // TODO: relocate((gep p, C, C2, ...)) -> gep(relocate(p), C, C2, ...)
+  }
   }
 
   return visitCallSite(II);

Added: llvm/trunk/test/Transforms/InstCombine/statepoint.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/statepoint.ll?rev=224968&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/statepoint.ll (added)
+++ llvm/trunk/test/Transforms/InstCombine/statepoint.ll Mon Dec 29 17:27:30 2014
@@ -0,0 +1,52 @@
+; RUN: opt < %s -instcombine -S | FileCheck %s
+; These tests check the optimizations specific to
+; pointers being relocated at a statepoint.
+
+
+declare void @func()
+
+define i1 @test_negative(i32 addrspace(1)* %p) {
+entry:
+  %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)* %p)
+  %pnew = call i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(i32 %safepoint_token, i32 4, i32 4)
+  %cmp = icmp eq i32 addrspace(1)* %pnew, null
+  ret i1 %cmp
+; CHECK-LABEL: test_negative
+; CHECK: %pnew = call i32 addrspace(1)*
+; CHECK: ret i1 %cmp
+}
+
+define i1 @test_nonnull(i32 addrspace(1)* nonnull %p) {
+entry:
+  %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)* %p)
+  %pnew = call i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(i32 %safepoint_token, i32 4, i32 4)
+  %cmp = icmp eq i32 addrspace(1)* %pnew, null
+  ret i1 %cmp
+; CHECK-LABEL: test_nonnull
+; CHECK: ret i1 false
+}
+
+define i1 @test_null() {
+entry:
+  %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)* null)
+  %pnew = call i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(i32 %safepoint_token, i32 4, i32 4)
+  %cmp = icmp eq i32 addrspace(1)* %pnew, null
+  ret i1 %cmp
+; CHECK-LABEL: test_null
+; CHECK-NOT: %pnew
+; CHECK: ret i1 true
+}
+
+define i1 @test_undef() {
+entry:
+  %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)* undef)
+  %pnew = call i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(i32 %safepoint_token, i32 4, i32 4)
+  %cmp = icmp eq i32 addrspace(1)* %pnew, null
+  ret i1 %cmp
+; CHECK-LABEL: test_undef
+; CHECK-NOT: %pnew
+; CHECK: ret i1 undef
+}
+
+declare i32 @llvm.experimental.gc.statepoint.p0f_isVoidf(void ()*, i32, i32, ...)
+declare i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(i32, i32, i32) #3