[Attributor] Ignore different kernels for kernel lifetime objects

If a potential interfering access is in a different kernel and the underlying object has kernel lifetime we can straight out ignore the interfering access. TODO: This should be made much stronger via "reaching kernels", which we already track in AAKernelInfo.
ilya-biryukov · Oct 21, 2023 · ba87fba · ba87fba
1 parent bb96093
commit ba87fba
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Showing 2 changed files with 20 additions and 7 deletions.
diff --git a/llvm/lib/Transforms/IPO/AttributorAttributes.cpp b/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
@@ -1200,6 +1200,11 @@ struct AAPointerInfoImpl
         A, this, IRPosition::function(Scope), DepClassTy::OPTIONAL,
         IsKnownNoRecurse);
 
+    // TODO: Use reaching kernels from AAKernelInfo (or move it to
+    // AAExecutionDomain) such that we allow scopes other than kernels as long
+    // as the reaching kernels are disjoint.
+    bool InstInKernel = Scope.hasFnAttribute("kernel");
+    bool ObjHasKernelLifetime = false;
     const bool UseDominanceReasoning =
         FindInterferingWrites && IsKnownNoRecurse;
     const DominatorTree *DT =
@@ -1232,6 +1237,7 @@ struct AAPointerInfoImpl
       // If the alloca containing function is not recursive the alloca
       // must be dead in the callee.
       const Function *AIFn = AI->getFunction();
+      ObjHasKernelLifetime = AIFn->hasFnAttribute("kernel");
       bool IsKnownNoRecurse;
       if (AA::hasAssumedIRAttr<Attribute::NoRecurse>(
               A, this, IRPosition::function(*AIFn), DepClassTy::OPTIONAL,
@@ -1241,7 +1247,8 @@ struct AAPointerInfoImpl
     } else if (auto *GV = dyn_cast<GlobalValue>(&getAssociatedValue())) {
       // If the global has kernel lifetime we can stop if we reach a kernel
       // as it is "dead" in the (unknown) callees.
-      if (HasKernelLifetime(GV, *GV->getParent()))
+      ObjHasKernelLifetime = HasKernelLifetime(GV, *GV->getParent());
+      if (ObjHasKernelLifetime)
         IsLiveInCalleeCB = [](const Function &Fn) {
           return !Fn.hasFnAttribute("kernel");
         };
@@ -1252,6 +1259,15 @@ struct AAPointerInfoImpl
     AA::InstExclusionSetTy ExclusionSet;
 
     auto AccessCB = [&](const Access &Acc, bool Exact) {
+      Function *AccScope = Acc.getRemoteInst()->getFunction();
+      bool AccInSameScope = AccScope == &Scope;
+
+      // If the object has kernel lifetime we can ignore accesses only reachable
+      // by other kernels. For now we only skip accesses *in* other kernels.
+      if (InstInKernel && ObjHasKernelLifetime && !AccInSameScope &&
+          AccScope->hasFnAttribute("kernel"))
+        return true;
+
       if (Exact && Acc.isMustAccess() && Acc.getRemoteInst() != &I) {
         if (Acc.isWrite() || (isa<LoadInst>(I) && Acc.isWriteOrAssumption()))
           ExclusionSet.insert(Acc.getRemoteInst());
@@ -1262,8 +1278,7 @@ struct AAPointerInfoImpl
         return true;
 
       bool Dominates = FindInterferingWrites && DT && Exact &&
-                       Acc.isMustAccess() &&
-                       (Acc.getRemoteInst()->getFunction() == &Scope) &&
+                       Acc.isMustAccess() && AccInSameScope &&
                        DT->dominates(Acc.getRemoteInst(), &I);
       if (Dominates)
         DominatingWrites.insert(&Acc);

diff --git a/llvm/test/Transforms/Attributor/value-simplify-gpu.ll b/llvm/test/Transforms/Attributor/value-simplify-gpu.ll
@@ -401,8 +401,7 @@ define dso_local void @kernel3(i32 %C) norecurse "kernel" {
 ; TUNIT-NEXT:    [[I:%.*]] = icmp eq i32 [[C]], 42
 ; TUNIT-NEXT:    br i1 [[I]], label [[T:%.*]], label [[F:%.*]]
 ; TUNIT:       t:
-; TUNIT-NEXT:    [[L:%.*]] = load i32, ptr addrspace(3) @AS3OneKernelAtATime, align 4
-; TUNIT-NEXT:    call void @use(i32 noundef [[L]], i32 noundef [[L]], i32 noundef [[L]]) #[[ATTR7]]
+; TUNIT-NEXT:    call void @use(i32 noundef 42, i32 noundef 42, i32 noundef 42) #[[ATTR7]]
 ; TUNIT-NEXT:    ret void
 ; TUNIT:       f:
 ; TUNIT-NEXT:    ret void
@@ -413,8 +412,7 @@ define dso_local void @kernel3(i32 %C) norecurse "kernel" {
 ; CGSCC-NEXT:    [[I:%.*]] = icmp eq i32 [[C]], 42
 ; CGSCC-NEXT:    br i1 [[I]], label [[T:%.*]], label [[F:%.*]]
 ; CGSCC:       t:
-; CGSCC-NEXT:    [[L:%.*]] = load i32, ptr addrspace(3) @AS3OneKernelAtATime, align 4
-; CGSCC-NEXT:    call void @use(i32 noundef [[L]], i32 noundef [[L]], i32 noundef [[L]]) #[[ATTR4]]
+; CGSCC-NEXT:    call void @use(i32 noundef 42, i32 noundef 42, i32 noundef 42) #[[ATTR4]]
 ; CGSCC-NEXT:    ret void
 ; CGSCC:       f:
 ; CGSCC-NEXT:    ret void