[BasicAA] Deoptimize intrinsics don't modify memory

[apilipenko]

Summary

Similarly to assumes and guards deoptimize intrinsics are marked as writing to ensure proper control dependencies but they never modify any particular memory location.

In most cases aliasing of deoptimize calls doesn't matter because these calls never return to the compiled code. One case were it does matter is transformations of the deopt operand bundle attached to the deoptimize. According to the lang ref "operand bundles represent an alternate “safe” continuation for the call site they’re attached to". Semantically the deopt bundle values are used after the call. (This is true for any call with deopt operand bundle, not only deoptimize calls). When we rewrite the deopt bundle values we need to take the effects of the call into account.

llvm/lib/Analysis/BasicAliasAnalysis.cpp

ModRefInfo BasicAAResult::getModRefInfo(const CallBase *Call,
   // that proper control dependencies are maintained but they never mods any
   // particular memory location.
   //
   // *Unlike* assumes, guard intrinsics are modeled as reading memory since the
   // heap state at the point the guard is issued needs to be consistent in case
   // the guard invokes the "deopt" continuation.
   if (isIntrinsicCall(Call, Intrinsic::experimental_guard))
     return ModRefInfo::Ref;
+  // The same applies to deoptimize which is essentially a guard(false).
+  if (isIntrinsicCall(Call, Intrinsic::experimental_deoptimize))
+    return ModRefInfo::Ref;
 
   // Like assumes, invariant.start intrinsics were also marked as arbitrarily
   // writing so that proper control dependencies are maintained but they never
   // mod any particular memory location visible to the IR.
   // *Unlike* assumes (which are now modeled as NoModRef), invariant.start
   // intrinsic is now modeled as reading memory. This prevents hoisting the
   // invariant.start intrinsic over stores. Consider:
   // *ptr = 40;

llvm/test/Analysis/BasicAA/deoptimize.ll

+; RUN: opt < %s -basic-aa -aa-eval -print-all-alias-modref-info -disable-output 2>&1 | FileCheck %s
+target datalayout = "e-p:32:32:32-i1:8:32-i8:8:32-i16:16:32-i32:32:32-i64:32:32-f32:32:32-f64:32:32-v64:32:64-v128:32:128-a0:0:32-n32"
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) #0
+declare void @llvm.experimental.deoptimize.void(...)
+declare void @unknown_but_readonly() readonly
+
+define void @test1(i8* %p) {
+  call void(...) @llvm.experimental.deoptimize.void() [ "deopt"() ]
+  ret void
+
+; CHECK-LABEL: Function: test1:
+; CHECK:  Just Ref: Ptr: i8* %p <-> call void (...) @llvm.experimental.deoptimize.isVoid() [ "deopt"() ]
+}