[SCEV] Ensure SCEV does not replace aliases with their aliasees

[leonardchan]

Summary

Passes in general shouldn't replace an alias with the aliasee (see https://reviews.llvm.org/D66606). This can lead to situations where a linkonce_odr symbol (which could be interposable if lowered to weak linkage) can be replaced with a local aliasee which won't be interposable. SVEC does this when the function is invoked by FunctionPass Manager -> Loop Pass Manager -> Induction Variable Users in the codegen pipeline. This was found in hwasan instrumented code where a linonce_odr alias was replaced with its private aliasee.

This fixes the bug descriped at https://github.com/llvm/llvm-project/issues/60668.

[leonardchan]

Lemme know if the clang regression test isn't necessary or should be in its own commit.

[leonardchan]

*ping*

[pcc]

Passes shouldn't be replacing aliases with aliasees in general, see D66606. The right fix is to fix SCEV to not do that.

[leonardchan]

In D144035#4143030, @pcc wrote:

Passes shouldn't be replacing aliases with aliasees in general, see D66606. The right fix is to fix SCEV to not do that.

Updated

[MaskRay]

LG

clang/test/CodeGenCXX/pr60668.cpp

1 ↗

(On Diff #499349)

We try avoiding .cc to assembly test. Drop this file.

[MaskRay]

Full context at https://github.com/llvm/llvm-project/issues/60668.

Best to leave a simplified description in the summary so that a reader doesn't have to go to the external link and summarize the discussions themselves.
And give a link to https://reviews.llvm.org/D66606

This SCEV function is invoked by FunctionPass Manager -> Loop Pass Manager -> Induction Variable Users in the codegen pipeline.

[leonardchan]

In D144035#4145662, @MaskRay wrote:

Full context at https://github.com/llvm/llvm-project/issues/60668.

Best to leave a simplified description in the summary so that a reader doesn't have to go to the external link and summarize the discussions themselves.
And give a link to https://reviews.llvm.org/D66606

This SCEV function is invoked by FunctionPass Manager -> Loop Pass Manager -> Induction Variable Users in the codegen pipeline.

Updated

[MaskRay]

[llvm]

[SCEV] is a more common tag for ScalarEvolution patches.

[leonardchan]

@pcc does this lgty?

[pcc]

LGTM

[leonardchan]

@reames let's continue the discussion here. What specific bits with the optimizer does this break? I'm not too familiar with SVEC. Prior to this, one thing I tried was instead just doing Ops.push_back(GA) and getSCEV(GA) but those seemed to result in infinite recursions last I checked probably because there's another pass attempting to resolve aliases.

llvm/lib/Analysis/ScalarEvolution.cpp

if (Instruction *I = dyn_cast<Instruction>(V)) {
     // reachable. Such instructions don't matter, and they aren't required
     // to obey basic rules for definitions dominating uses which this
     // analysis depends on.
     if (!DT.isReachableFromEntry(I->getParent()))
       return getUnknown(PoisonValue::get(V->getType()));
   } else if (ConstantInt *CI = dyn_cast<ConstantInt>(V))
     return getConstant(CI);
   else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V)) {
-    if (!GA->isInterposable()) {
-      Ops.push_back(GA->getAliasee());
-      return nullptr;
-    }
     return getUnknown(V);
   } else if (!isa<ConstantExpr>(V))
     return getUnknown(V);
 
   Operator *U = cast<Operator>(V);
   if (auto BO =
           MatchBinaryOp(U, getDataLayout(), AC, DT, dyn_cast<Instruction>(V))) {
     bool IsConstArg = isa<ConstantInt>(BO->RHS);
if (Instruction *I = dyn_cast<Instruction>(V)) {
     // reachable. Such instructions don't matter, and they aren't required
     // to obey basic rules for definitions dominating uses which this
     // analysis depends on.
     if (!DT.isReachableFromEntry(I->getParent()))
       return getUnknown(PoisonValue::get(V->getType()));
   } else if (ConstantInt *CI = dyn_cast<ConstantInt>(V))
     return getConstant(CI);
   else if (GlobalAlias *GA = dyn_cast<GlobalAlias>(V))
-    return GA->isInterposable() ? getUnknown(V) : getSCEV(GA->getAliasee());
+    return getUnknown(V);
   else if (!isa<ConstantExpr>(V))
     return getUnknown(V);
 
   const SCEV *LHS;
   const SCEV *RHS;
 
   Operator *U = cast<Operator>(V);
   if (auto BO =

llvm/test/Analysis/ScalarEvolution/no-follow-alias.ll

+; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py
+; RUN: opt -passes='print<scalar-evolution>' -disable-output %s 2>&1 | FileCheck %s
+
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-unknown-linux-gnu"
+
[e-mail] = private constant [32 x i32] zeroinitializer
+@glob = linkonce_odr hidden alias [32 x i32], inttoptr (i64 add (i64 ptrtoint (ptr @glob.private to i64), i64 1234) to ptr)
+
+define hidden void @func(i64 %idx) local_unnamed_addr {
+; CHECK-LABEL: 'func'
+; CHECK-NEXT:  Classifying expressions for: @func
+; CHECK-NEXT:    %arrayidx = getelementptr inbounds [32 x i32], ptr @glob, i64 0, i64 %idx
+; CHECK-NEXT:    --> ((4 * %idx) + @glob) U: [0,-1) S: [-9223372036854775808,9223372036854775807)
+; CHECK-NEXT:  Determining loop execution counts for: @func
+;
+  %arrayidx = getelementptr inbounds [32 x i32], ptr @glob, i64 0, i64 %idx
+  ret void
+}