[ArgPromotion] Perform alias analysis on actual arguments of Calls

Teach Argument Promotion to perform alias analysis on actual arguments
of Calls to a Function, to try to prove that all Calls to the Function
do not modify the memory pointed to by an argument. This surfaces more
opportunities to perform Argument Promotion in cases where simply
looking at a Function's instructions is insufficient to prove that the
pointer argument is not invalidated before all loads from it.

Author: hazzlim

llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

@@ -474,7 +474,8 @@ static bool allCallersPassValidPointerForArgument(
 /// parts it can be promoted into.
 static bool findArgParts(Argument *Arg, const DataLayout &DL, AAResults &AAR,
                          unsigned MaxElements, bool IsRecursive,
-                         SmallVectorImpl<OffsetAndArgPart> &ArgPartsVec) {
+                         SmallVectorImpl<OffsetAndArgPart> &ArgPartsVec,
+                         bool ArgNotModified) {
   // Quick exit for unused arguments
   if (Arg->use_empty())
     return true;
@@ -696,8 +697,10 @@ static bool findArgParts(Argument *Arg, const DataLayout &DL, AAResults &AAR,
 
   // If store instructions are allowed, the path from the entry of the function
   // to each load may be not free of instructions that potentially invalidate
-  // the load, and this is an admissible situation.
-  if (AreStoresAllowed)
+  // the load, and this is an admissible situation. If we have already
+  // determined that the pointer Arg is not modified in the function (for all
+  // Calls) then we can similarly conclude analysis here.
+  if (AreStoresAllowed || ArgNotModified)
     return true;
 
   // Okay, now we know that the argument is only used by load instructions, and
@@ -745,6 +748,33 @@ static bool areTypesABICompatible(ArrayRef<Type *> Types, const Function &F,
   });
 }
 
+// Try to prove that all Calls to F do not modify the memory pointed to by Arg.
+// This can provide us with more opportunities to perform Argument Promotion in
+// cases where simply looking at a Function's instructions is insufficient to
+// prove that the pointer argument is not invalidated before all loads from it.
+static bool callDoesNotModifyArg(Function *F, unsigned ArgNo,
+                                 FunctionAnalysisManager &FAM) {
+  // Find all Users of F that are Calls, and see if they may modify Arg.
+  for (User *U : F->users()) {
+    auto *Call = dyn_cast<CallInst>(U);
+    if (!Call)
+      continue;
+
+    Value *ArgOp = Call->getArgOperand(ArgNo);
+    assert(ArgOp->getType()->isPointerTy() && "Argument must be Pointer Type!");
+
+    MemoryLocation Loc = MemoryLocation::getForArgument(Call, ArgNo, nullptr);
+
+    AAResults &AAR = FAM.getResult<AAManager>(*Call->getFunction());
+    // Bail out as soon as we find a Call where Arg may be modified.
+    if (isModSet(AAR.getModRefInfo(Call, Loc)))
+      return false;
+  }
+
+  // All Calls do not modify the Arg.
+  return true;
+}
+
 /// PromoteArguments - This method checks the specified function to see if there
 /// are any promotable arguments and if it is safe to promote the function (for
 /// example, all callers are direct).  If safe to promote some arguments, it
@@ -775,11 +805,13 @@ static Function *promoteArguments(Function *F, FunctionAnalysisManager &FAM,
     return nullptr;
 
   // First check: see if there are any pointer arguments!  If not, quick exit.
-  SmallVector<Argument *, 16> PointerArgs;
-  for (Argument &I : F->args())
-    if (I.getType()->isPointerTy())
-      PointerArgs.push_back(&I);
-  if (PointerArgs.empty())
+  SmallVector<unsigned, 16> PointerArgNos;
+  for (unsigned I = 0; I < F->arg_size(); ++I) {
+    Argument *Arg = F->getArg(I);
+    if (Arg->getType()->isPointerTy())
+      PointerArgNos.push_back(I);
+  }
+  if (PointerArgNos.empty())
     return nullptr;
 
   // Second check: make sure that all callers are direct callers.  We can't
@@ -814,7 +846,8 @@ static Function *promoteArguments(Function *F, FunctionAnalysisManager &FAM,
   // add it to ArgsToPromote.
   DenseMap<Argument *, SmallVector<OffsetAndArgPart, 4>> ArgsToPromote;
   unsigned NumArgsAfterPromote = F->getFunctionType()->getNumParams();
-  for (Argument *PtrArg : PointerArgs) {
+  for (const auto &ArgIdx : PointerArgNos) {
+    Argument *PtrArg = F->getArg(ArgIdx);
     // Replace sret attribute with noalias. This reduces register pressure by
     // avoiding a register copy.
     if (PtrArg->hasStructRetAttr()) {
@@ -828,10 +861,15 @@ static Function *promoteArguments(Function *F, FunctionAnalysisManager &FAM,
       }
     }
 
+    // Check if we can determine ahead of time that the argument is never
+    // modified by a call to this function.
+    bool ArgNotModified = callDoesNotModifyArg(F, ArgIdx, FAM);
+
     // If we can promote the pointer to its value.
     SmallVector<OffsetAndArgPart, 4> ArgParts;
 
-    if (findArgParts(PtrArg, DL, AAR, MaxElements, IsRecursive, ArgParts)) {
+    if (findArgParts(PtrArg, DL, AAR, MaxElements, IsRecursive, ArgParts,
+                     ArgNotModified)) {
       SmallVector<Type *, 4> Types;
       for (const auto &Pair : ArgParts)
         Types.push_back(Pair.second.Ty);

llvm/test/Transforms/ArgumentPromotion/actual-arguments.ll

@@ -75,11 +75,9 @@ define internal i32 @test_cannot_promote_3(ptr %p, ptr nocapture readonly %test_
 ;
 define internal i32 @test_can_promote_1(ptr %p, ptr nocapture readonly %test_c) {
 ; CHECK-LABEL: define {{[^@]+}}@test_can_promote_1
-; CHECK-SAME: (ptr [[P:%.*]], ptr nocapture readonly [[TEST_C:%.*]]) {
-; CHECK-NEXT:    [[TEST_C_VAL:%.*]] = load i32, ptr [[TEST_C]], align 4
-; CHECK-NEXT:    [[RES:%.*]] = call i32 @callee(ptr [[P]], i32 [[TEST_C_VAL]])
-; CHECK-NEXT:    [[LTEST_C:%.*]] = load i32, ptr [[TEST_C]], align 4
-; CHECK-NEXT:    [[SUM:%.*]] = add i32 [[LTEST_C]], [[RES]]
+; CHECK-SAME: (ptr [[P:%.*]], i32 [[TEST_C_0_VAL:%.*]]) {
+; CHECK-NEXT:    [[RES:%.*]] = call i32 @callee(ptr [[P]], i32 [[TEST_C_0_VAL]])
+; CHECK-NEXT:    [[SUM:%.*]] = add i32 [[TEST_C_0_VAL]], [[RES]]
 ; CHECK-NEXT:    ret i32 [[SUM]]
 ;
   %res = call i32 @callee(ptr %p, ptr %test_c)
@@ -99,11 +97,9 @@ define internal i32 @test_can_promote_1(ptr %p, ptr nocapture readonly %test_c)
 ;
 define internal i32 @test_can_promote_2(ptr %p, ptr nocapture readonly %test_c) {
 ; CHECK-LABEL: define {{[^@]+}}@test_can_promote_2
-; CHECK-SAME: (ptr [[P:%.*]], ptr nocapture readonly [[TEST_C:%.*]]) {
-; CHECK-NEXT:    [[TEST_C_VAL:%.*]] = load i32, ptr [[TEST_C]], align 4
-; CHECK-NEXT:    [[RES:%.*]] = call i32 @callee(ptr [[P]], i32 [[TEST_C_VAL]])
-; CHECK-NEXT:    [[LTEST_C:%.*]] = load i32, ptr [[TEST_C]], align 4
-; CHECK-NEXT:    [[SUM:%.*]] = add i32 [[LTEST_C]], [[RES]]
+; CHECK-SAME: (ptr [[P:%.*]], i32 [[TEST_C_0_VAL:%.*]]) {
+; CHECK-NEXT:    [[RES:%.*]] = call i32 @callee(ptr [[P]], i32 [[TEST_C_0_VAL]])
+; CHECK-NEXT:    [[SUM:%.*]] = add i32 [[TEST_C_0_VAL]], [[RES]]
 ; CHECK-NEXT:    ret i32 [[SUM]]
 ;
   %res = call i32 @callee(ptr %p, ptr %test_c)
@@ -186,8 +182,10 @@ define i32 @caller_safe_args_1(i64 %n) {
 ; CHECK-NEXT:    [[CALLER_C:%.*]] = alloca i32, align 4
 ; CHECK-NEXT:    store i32 5, ptr [[CALLER_C]], align 4
 ; CHECK-NEXT:    [[RES1:%.*]] = call i32 @test_cannot_promote_3(ptr [[P]], ptr [[CALLER_C]])
-; CHECK-NEXT:    [[RES2:%.*]] = call i32 @test_can_promote_1(ptr [[P]], ptr [[CALLER_C]])
-; CHECK-NEXT:    [[RES3:%.*]] = call i32 @test_can_promote_2(ptr [[P]], ptr [[CALLER_C]])
+; CHECK-NEXT:    [[CALLER_C_VAL:%.*]] = load i32, ptr [[CALLER_C]], align 4
+; CHECK-NEXT:    [[RES2:%.*]] = call i32 @test_can_promote_1(ptr [[P]], i32 [[CALLER_C_VAL]])
+; CHECK-NEXT:    [[CALLER_C_VAL1:%.*]] = load i32, ptr [[CALLER_C]], align 4
+; CHECK-NEXT:    [[RES3:%.*]] = call i32 @test_can_promote_2(ptr [[P]], i32 [[CALLER_C_VAL1]])
 ; CHECK-NEXT:    [[RES12:%.*]] = add i32 [[RES1]], [[RES2]]
 ; CHECK-NEXT:    [[RES:%.*]] = add i32 [[RES12]], [[RES3]]
 ; CHECK-NEXT:    ret i32 [[RES]]
@@ -215,7 +213,8 @@ define i32 @caller_safe_args_2(i64 %n, ptr %p) {
 ; CHECK-NEXT:    call void @memset(ptr [[P]], i64 0, i64 [[N]])
 ; CHECK-NEXT:    [[CALLER_C:%.*]] = alloca i32, align 4
 ; CHECK-NEXT:    store i32 5, ptr [[CALLER_C]], align 4
-; CHECK-NEXT:    [[RES:%.*]] = call i32 @test_can_promote_2(ptr [[P]], ptr [[CALLER_C]])
+; CHECK-NEXT:    [[CALLER_C_VAL:%.*]] = load i32, ptr [[CALLER_C]], align 4
+; CHECK-NEXT:    [[RES:%.*]] = call i32 @test_can_promote_2(ptr [[P]], i32 [[CALLER_C_VAL]])
 ; CHECK-NEXT:    ret i32 [[RES]]
 ;
   call void @memset(ptr %p, i64 0, i64 %n)