From 2fbabf6e6dc61e0627b93a596bf13e272345c5e8 Mon Sep 17 00:00:00 2001
From: Matthijs Kooijman <matthijs@stdin.nl>
Date: Tue, 17 Jun 2008 12:20:24 +0000
Subject: [PATCH] Learn IPConstProp to propagate arguments that are directly
 returned. Strictly speaking these are not constant values. However, when a
 function always returns one of its arguments, then from the point of view of
 each caller the return value is constant (or at least a known value) and can
 be replaced.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@52397 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Transforms/IPO/IPConstantPropagation.cpp | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/lib/Transforms/IPO/IPConstantPropagation.cpp b/lib/Transforms/IPO/IPConstantPropagation.cpp
index 0e654a50eef..54127f6a7ae 100644
--- a/lib/Transforms/IPO/IPConstantPropagation.cpp
+++ b/lib/Transforms/IPO/IPConstantPropagation.cpp
@@ -145,6 +145,10 @@ bool IPCP::PropagateConstantsIntoArguments(Function &F) {
 // all callers that use those return values with the constant value. This will
 // leave in the actual return values and instructions, but deadargelim will
 // clean that up.
+//
+// Additionally if a function always returns one of its arguments directly,
+// callers will be updated to use the value they pass in directly instead of
+// using the return value.
 bool IPCP::PropagateConstantReturn(Function &F) {
   if (F.getReturnType() == Type::VoidTy)
     return false; // No return value.
@@ -188,8 +192,8 @@ bool IPCP::PropagateConstantReturn(Function &F) {
           if (isa<UndefValue>(V))
             continue;
           
-          // Try to see if all the rets return the same constant.
-          if (isa<Constant>(V)) {
+          // Try to see if all the rets return the same constant or argument.
+          if (isa<Constant>(V) || isa<Argument>(V)) {
             if (isa<UndefValue>(RV)) {
               // No value found yet? Try the current one.
               RetVals[i] = V;
@@ -255,6 +259,11 @@ bool IPCP::PropagateConstantReturn(Function &F) {
       if (index != -1) {
         Value *New = RetVals[index];
         if (New) {
+          if (Argument *A = dyn_cast<Argument>(New))
+            // Was an argument returned? Then find the corresponding argument in
+            // the call instruction and use that. Add 1 to the argument number
+            // to skipp the first argument (the function itself).
+            New = Call->getOperand(A->getArgNo() + 1);
           Ins->replaceAllUsesWith(New);
           Ins->eraseFromParent();
         }