[CGP] Undo constant propagation of pointers across calls

It may be profitable to revert SCCP propagation of C++ static values,
if such constants are pointers, in order to avoid redundant pointer
computation, since the method returning the constant is non-removable.

Author: antoniofrighetto

llvm/lib/CodeGen/CodeGenPrepare.cpp

@@ -2677,7 +2677,8 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT) {
   }
 
   // From here on out we're working with named functions.
-  if (!CI->getCalledFunction())
+  auto *Callee = CI->getCalledFunction();
+  if (!Callee)
     return false;
 
   // Lower all default uses of _chk calls.  This is very similar
@@ -2692,6 +2693,51 @@ bool CodeGenPrepare::optimizeCallInst(CallInst *CI, ModifyDT &ModifiedDT) {
     return true;
   }
 
+  // SCCP may have propagated, among other things, C++ static variables across
+  // calls. If this happens to be the case, we may want to undo it in order to
+  // avoid redundant pointer computation of the constant, as the function method
+  // returning the constant needs to be executed anyways.
+  auto GetUniformReturnValue = [](const Function *F) -> GlobalVariable * {
+    if (!F->getReturnType()->isPointerTy())
+      return nullptr;
+
+    GlobalVariable *UniformValue = nullptr;
+    for (auto &BB : *F) {
+      if (auto *RI = dyn_cast<ReturnInst>(BB.getTerminator())) {
+        if (auto *V = dyn_cast<GlobalVariable>(RI->getReturnValue())) {
+          if (!UniformValue)
+            UniformValue = V;
+          else if (V != UniformValue)
+            return nullptr;
+        } else {
+          return nullptr;
+        }
+      }
+    }
+
+    return UniformValue;
+  };
+
+  if (Callee->hasExactDefinition()) {
+    if (GlobalVariable *RV = GetUniformReturnValue(Callee)) {
+      bool MadeChange = false;
+      for (Use &U : make_early_inc_range(RV->uses())) {
+        auto *I = dyn_cast<Instruction>(U.getUser());
+        if (!I || I->getParent() != CI->getParent()) {
+          // Limit to the same basic block to avoid extending the call-site live
+          // range, which otherwise could increase register pressure.
+          continue;
+        }
+        if (CI->comesBefore(I)) {
+          U.set(CI);
+          MadeChange = true;
+        }
+      }
+
+      return MadeChange;
+    }
+  }
+
   return false;
 }
 

llvm/test/Transforms/CodeGenPrepare/revert-constant-ptr-propagation-on-calls.ll

@@ -60,7 +60,7 @@ entry:
 define i32 @caller_1() {
 ; CHECK-LABEL: @caller_1(
 ; CHECK-NEXT:    [[GETS_PTR:%.*]] = call ptr @getS()
-; CHECK-NEXT:    [[GETI:%.*]] = call i32 @S_getI(ptr @g_getS)
+; CHECK-NEXT:    [[GETI:%.*]] = call i32 @S_getI(ptr [[GETS_PTR]])
 ; CHECK-NEXT:    ret i32 [[GETI]]
 ;
   %getS_ptr = call ptr @getS()