[GlobalOpt] recognize dead struct fields and propagate values

Summary:
Allow struct fields SRA and dead stores. This works by considering fields accesses from getElementPtr to be considered as a possible pointer root that can be cleaned up.
We check that the variable can be SRA by recursively checking the sub expressions with the new isSafeSubSROAGEP function.

basically this allows the array in following C code  to be optimized out 

struct Expr {
  int a[2];
  int b;
};

static struct Expr e;

int foo (int i)
{
  e.b = 2;
  e.a[i] = 1;
  return e.b;
}


Reviewers: greened, bkramer, nicholas, jmolloy

Reviewed By: jmolloy

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61911

llvm-svn: 361460

Author: Christian Bruel

llvm/lib/Transforms/IPO/GlobalOpt.cpp

@@ -184,7 +184,7 @@ static bool IsSafeComputationToRemove(Value *V, const TargetLibraryInfo *TLI) {
 /// This GV is a pointer root.  Loop over all users of the global and clean up
 /// any that obviously don't assign the global a value that isn't dynamically
 /// allocated.
-static bool CleanupPointerRootUsers(GlobalVariable *GV,
+static bool CleanupPointerRootUsers(Value *V,
                                     const TargetLibraryInfo *TLI) {
   // A brief explanation of leak checkers.  The goal is to find bugs where
   // pointers are forgotten, causing an accumulating growth in memory
@@ -202,7 +202,7 @@ static bool CleanupPointerRootUsers(GlobalVariable *GV,
   SmallVector<std::pair<Instruction *, Instruction *>, 32> Dead;
 
   // Constants can't be pointers to dynamically allocated memory.
-  for (Value::user_iterator UI = GV->user_begin(), E = GV->user_end();
+  for (Value::user_iterator UI = V->user_begin(), E = V->user_end();
        UI != E;) {
     User *U = *UI++;
     if (StoreInst *SI = dyn_cast<StoreInst>(U)) {
@@ -232,6 +232,9 @@ static bool CleanupPointerRootUsers(GlobalVariable *GV,
           Dead.push_back(std::make_pair(I, MTI));
       }
     } else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(U)) {
+      if (CE->getOpcode() == Instruction::GetElementPtr) {
+        Changed |= CleanupPointerRootUsers(CE, TLI);
+      }
       if (CE->use_empty()) {
         CE->destroyConstant();
         Changed = true;
@@ -241,7 +244,7 @@ static bool CleanupPointerRootUsers(GlobalVariable *GV,
         C->destroyConstant();
         // This could have invalidated UI, start over from scratch.
         Dead.clear();
-        CleanupPointerRootUsers(GV, TLI);
+        CleanupPointerRootUsers(V, TLI);
         return true;
       }
     }
@@ -391,6 +394,22 @@ static bool isSafeSROAGEP(User *U) {
                       [](User *UU) { return isSafeSROAElementUse(UU); });
 }
 
+/// Return true if the specified GEP is a safe user of a derived
+/// expression from a global that we want to SROA.
+static bool isSafeSubSROAGEP(User *U) {
+
+  // Check to see if this ConstantExpr GEP is SRA'able.  In particular, we
+  // don't like < 3 operand CE's, and we don't like non-constant integer
+  // indices.  This enforces that all uses are 'gep GV, 0, C, ...' for some
+  // value of C.
+  if (U->getNumOperands() < 3 || !isa<Constant>(U->getOperand(1)) ||
+      !cast<Constant>(U->getOperand(1))->isNullValue())
+    return false;
+
+  return llvm::all_of(U->users(),
+                      [](User *UU) { return isSafeSROAElementUse(UU); });
+}
+
 /// Return true if the specified instruction is a safe user of a derived
 /// expression from a global that we want to SROA.
 static bool isSafeSROAElementUse(Value *V) {
@@ -409,7 +428,7 @@ static bool isSafeSROAElementUse(Value *V) {
     return SI->getOperand(0) != V;
 
   // Otherwise, it must be a GEP. Check it and its users are safe to SRA.
-  return isa<GetElementPtrInst>(I) && isSafeSROAGEP(I);
+  return isa<GetElementPtrInst>(I) && isSafeSubSROAGEP(I);
 }
 
 /// Look at all uses of the global and decide whether it is safe for us to

llvm/test/Transforms/GlobalOpt/globalsra-multigep.ll

@@ -4,13 +4,20 @@ target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 @g_data = internal unnamed_addr global <{ [8 x i16], [8 x i16] }> <{ [8 x i16] [i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16], [8 x i16] zeroinitializer }>, align 16
-; We cannot SRA here due to the second gep meaning the access to g_data may be to either element
-; CHECK: @g_data = internal unnamed_addr constant <{ [8 x i16], [8 x i16] }>
+; We normally cannot SRA here due to the second gep meaning the access to g_data may be to either element,
+; unless the value is always zero.
+; CHECK: @g_data.0 = internal unnamed_addr constant [8 x i16] [i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16, i16 16], align 16
 
 define i16 @test(i64 %a1) {
 entry:
   %g1 = getelementptr inbounds <{ [8 x i16], [8 x i16] }>, <{ [8 x i16], [8 x i16] }>* @g_data, i64 0, i32 0
   %arrayidx.i = getelementptr inbounds [8 x i16], [8 x i16]* %g1, i64 0, i64 %a1
   %r = load i16, i16* %arrayidx.i, align 2
+
+; CHECK-NOT: getelementptr inbounds <{ [8 x i16], [8 x i16] }>, <{ [8 x i16], [8 x i16] }>* @g_data, i64 0, i32 0
+; CHECK:  %arrayidx.i = getelementptr inbounds [8 x i16], [8 x i16]* @g_data.0, i64 0, i64 %a1
+
   ret i16 %r
+
+
 }

llvm/test/Transforms/GlobalOpt/globalsra-struct.ll

@@ -0,0 +1,18 @@
+; RUN: opt < %s -globalopt -S | FileCheck %s
+
+%struct.Expr = type { [1 x i32], i32 }
+
+@e = internal global %struct.Expr zeroinitializer, align 4
+; CHECK-NOT: @e = internal global %struct.Expr zeroinitializer, align 4
+
+define dso_local i32 @foo(i32 %i) {
+entry:
+  %i.addr = alloca i32, align 4
+  store i32 %i, i32* %i.addr, align 4
+  %0 = load i32, i32* %i.addr, align 4
+  %arrayidx = getelementptr inbounds [1 x i32], [1 x i32]* getelementptr inbounds (%struct.Expr, %struct.Expr* @e, i32 0, i32 0), i32 0, i32 %0
+  store i32 57005, i32* %arrayidx, align 4
+  %1 = load i32, i32* getelementptr inbounds (%struct.Expr, %struct.Expr* @e, i32 0, i32 1), align 4
+  ret i32 %1
+; CHECK:  ret i32 0
+}