Index: lib/Transforms/Utils/SimplifyLibCalls.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -1834,9 +1834,9 @@
   // printf("x") -> putchar('x'), even for '%'.
   if (FormatStr.size() == 1) {
     Value *Res = emitPutChar(B.getInt32(FormatStr[0]), B, TLI);
-    if (CI->use_empty() || !Res)
-      return Res;
-    return B.CreateIntCast(Res, CI->getType(), true);
+    assert(CI->use_empty() &&
+           "printf return value used should inhibit optimizations");
+    return Res;
   }
 
   // printf("%s", "a") --> putchar('a')
@@ -1847,14 +1847,9 @@
     if (ChrStr.size() != 1)
       return nullptr;
     Value *Res = emitPutChar(B.getInt32(ChrStr[0]), B, TLI);
-
-    // FIXME: Here we check that the return value is not used
-    // but ealier we prevent transformations in case it is.
-    // This should probably be an assert.
-    if (CI->use_empty() || !Res)
-      return Res;
-
-    return B.CreateIntCast(Res, CI->getType(), true);
+    assert(CI->use_empty() &&
+           "printf return value used should inhibit optimizations");
+    return Res;
   }
 
   // printf("foo\n") --> puts("foo")
@@ -1865,9 +1860,9 @@
     FormatStr = FormatStr.drop_back();
     Value *GV = B.CreateGlobalString(FormatStr, "str");
     Value *NewCI = emitPutS(GV, B, TLI);
-    return (CI->use_empty() || !NewCI)
-               ? NewCI
-               : ConstantInt::get(CI->getType(), FormatStr.size() + 1);
+    assert(CI->use_empty() &&
+           "printf return value used should inhibit optimizations");
+    return NewCI;
   }
 
   // Optimize specific format strings.
@@ -1875,16 +1870,18 @@
   if (FormatStr == "%c" && CI->getNumArgOperands() > 1 &&
       CI->getArgOperand(1)->getType()->isIntegerTy()) {
     Value *Res = emitPutChar(CI->getArgOperand(1), B, TLI);
-
-    if (CI->use_empty() || !Res)
-      return Res;
-    return B.CreateIntCast(Res, CI->getType(), true);
+    assert(CI->use_empty() &&
+           "printf return value used should inhibit optimizations");
+    return Res;
   }
 
   // printf("%s\n", str) --> puts(str)
   if (FormatStr == "%s\n" && CI->getNumArgOperands() > 1 &&
       CI->getArgOperand(1)->getType()->isPointerTy()) {
-    return emitPutS(CI->getArgOperand(1), B, TLI);
+    Value *Res = emitPutS(CI->getArgOperand(1), B, TLI);
+    assert(CI->use_empty() &&
+           "printf return value used should inhibit optimizations");
+    return Res;
   }
   return nullptr;
 }