[SimplifyCFG] Don't lower complex ConstantExprs to lookup tables

[olista01]

Summary

Not all ConstantExprs can be represented by a global variable, for example most pointer arithmetic other than addition of a constant, so we can't convert these values from switch statements to lookup tables.

[efriedma]

I don't think this testcase is quite right... it passes even without your patch. Maybe you need to tweak the GEP a bit so it doesn't fail the isGEPWithNoNotionalOverIndexing check?

[olista01]

Add a triple to the test, and move it to the ARM directory. Previously, the transformation was never done because a TTI check for legal types fails with the default implementation.

[efriedma]

LGTM.

lib/Transforms/Utils/SimplifyCFG.cpp

static bool ValidLookupTableConstant(Constant *C, const TargetTransformInfo &TTI) {
   if (C->isDLLImportDependent())
     return false;
 
   if (!isa<ConstantFP>(C) && !isa<ConstantInt>(C) &&
       !isa<ConstantPointerNull>(C) && !isa<GlobalValue>(C) &&
       !isa<UndefValue>(C) && !isa<ConstantExpr>(C))
     return false;
 
-  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C))
+  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(C)) {
     if (!CE->isGEPWithNoNotionalOverIndexing())
       return false;
+    if (!ValidLookupTableConstant(CE->getOperand(0), TTI))
+      return false;
+  }
 
   if (!TTI.shouldBuildLookupTablesForConstant(C))
     return false;
 
   return true;
 }
 
 /// If V is a Constant, return it. Otherwise, try to look up

test/Transforms/SimplifyCFG/ARM/switch-to-lookup-table-constant-expr.ll

+; RUN: opt -S -simplifycfg < %s | FileCheck %s --check-prefix=CHECK
+
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7a--none-eabi"
+
+; One of the phi node's values is too complex to be represented by a global
+; variable, so we can't convert to a lookup table.
+
+; CHECK-NOT: @switch.table
+; CHECK-NOT: load
+
+@g1 = external global i32, align 4
+@g2 = external global i32, align 4
+@g3 = external global i32, align 4
+@g4 = external thread_local global i32, align 4
+
+define i32* @test3(i32 %n) {
+entry:
+  switch i32 %n, label %sw.default [
+    i32 0, label %sw.bb
+    i32 1, label %sw.bb1
+    i32 2, label %sw.bb2
+  ]
+
+sw.bb:
+  br label %return
+
+sw.bb1:
+  br label %return
+
+sw.bb2:
+  br label %return
+
+sw.default:
+  br label %return
+
+return:
+  %retval.0 = phi i32* [ @g4, %sw.default ], [ getelementptr inbounds (i32, i32* inttoptr (i32 mul (i32 ptrtoint (i32* @g3 to i32), i32 2) to i32*), i32 1), %sw.bb2 ], [ @g2, %sw.bb1 ], [ @g1, %sw.bb ]
+  ret i32* %retval.0
+}