From 81778322300abd0bbad923185b2da8b3572e3597 Mon Sep 17 00:00:00 2001
From: Matthijs Kooijman <matthijs@stdin.nl>
Date: Wed, 18 Jun 2008 08:30:37 +0000
Subject: [PATCH] Reapply r52397 (make IPConstProp promote returned arguments),
 but fixed this time. Sorry for the trouble!

This time, also add a testcase, which I should have done in the first place...

llvm-svn: 52455
---
 lib/Transforms/IPO/IPConstantPropagation.cpp  | 21 +++++++--
 .../IPConstantProp/return-argument.ll         | 46 +++++++++++++++++++
 2 files changed, 64 insertions(+), 3 deletions(-)
 create mode 100644 test/Transforms/IPConstantProp/return-argument.ll

diff --git a/lib/Transforms/IPO/IPConstantPropagation.cpp b/lib/Transforms/IPO/IPConstantPropagation.cpp
index 0e654a50eef..f528b88983b 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;
@@ -227,7 +231,13 @@ bool IPCP::PropagateConstantReturn(Function &F) {
     MadeChange = true;
 
     if (STy == 0) {
-      Call->replaceAllUsesWith(RetVals[0]);
+      Value* New = RetVals[0];
+      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 skip the first argument (the function itself).
+        New = Call->getOperand(A->getArgNo() + 1);
+      Call->replaceAllUsesWith(New);
       continue;
     }
    
@@ -255,6 +265,11 @@ 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 skip the first argument (the function itself).
+            New = Call->getOperand(A->getArgNo() + 1);
           Ins->replaceAllUsesWith(New);
           Ins->eraseFromParent();
         }
diff --git a/test/Transforms/IPConstantProp/return-argument.ll b/test/Transforms/IPConstantProp/return-argument.ll
new file mode 100644
index 00000000000..53288ee1086
--- /dev/null
+++ b/test/Transforms/IPConstantProp/return-argument.ll
@@ -0,0 +1,46 @@
+; RUN: llvm-as < %s | opt -ipconstprop | llvm-dis > %t
+; RUN: cat %t | grep {store i32 %Z, i32\\* %Q}
+; RUN: cat %t | grep {add i32 1, 3}
+
+;; This function returns its second argument on all return statements
+define internal i32* @incdec(i1 %C, i32* %V) {
+        %X = load i32* %V
+        br i1 %C, label %T, label %F
+
+T:              ; preds = %0
+        %X1 = add i32 %X, 1
+        store i32 %X1, i32* %V
+        ret i32* %V
+
+F:              ; preds = %0
+        %X2 = sub i32 %X, 1
+        store i32 %X2, i32* %V
+        ret i32* %V
+}
+
+;; This function returns its first argument as a part of a multiple return
+;; value
+define internal { i32, i32 } @foo(i32 %A, i32 %B) {
+        %X = add i32 %A, %B
+        %Y = insertvalue { i32, i32 } undef, i32 %A, 0
+        %Z = insertvalue { i32, i32 } %Y, i32 %X, 1
+        ret { i32, i32 } %Z
+}
+
+define void @caller(i1 %C) {
+        %Q = alloca i32
+        ;; Call incdec to see if %W is properly replaced by %Q
+        %W = call i32* @incdec(i1 %C, i32* %Q )             ; <i32> [#uses=1]
+        ;; Call @foo twice, to prevent the arguments from propagating into the
+        ;; function (so we can check the returned argument is properly
+        ;; propagated per-caller).
+        %S1 = call { i32, i32 } @foo(i32 1, i32 2);
+        %X1 = extractvalue { i32, i32 } %S1, 0
+        %S2 = call { i32, i32 } @foo(i32 3, i32 4);
+        %X2 = extractvalue { i32, i32 } %S2, 0
+        ;; Do some stuff with the returned values which we can grep for
+        %Z  = add i32 %X1, %X2
+        store i32 %Z, i32* %W
+        ret void
+}
+