[PATCH] D14363: [FunctionAttrs] Remove a loop, NFC refactor

[Sanjoy Das via llvm-commits]

sanjoy created this revision.
sanjoy added reviewers: reames, chandlerc, nlewycky.
sanjoy added a subscriber: llvm-commits.

Remove the loop over the uses of the CallSite in ArgumentUsesTracker.
Since we have the `Use *` for actual argument operand, we can just use
pointer subtraction.

The time complexity remains the same though (except for a vararg
argument) -- `std::advance` is O(UseIndex) for the ArgumentList
iterator.

The real motivation is to make a later change adding support for operand
bundles simpler.

http://reviews.llvm.org/D14363

Files:
  lib/Transforms/IPO/FunctionAttrs.cpp

Index: lib/Transforms/IPO/FunctionAttrs.cpp
===================================================================
--- lib/Transforms/IPO/FunctionAttrs.cpp
+++ lib/Transforms/IPO/FunctionAttrs.cpp
@@ -321,23 +321,19 @@
       return true;
     }
 
-    bool Found = false;
-    Function::arg_iterator AI = F->arg_begin(), AE = F->arg_end();
-    for (CallSite::arg_iterator PI = CS.arg_begin(), PE = CS.arg_end();
-         PI != PE; ++PI, ++AI) {
-      if (AI == AE) {
-        assert(F->isVarArg() && "More params than args in non-varargs call");
-        Captured = true;
-        return true;
-      }
-      if (PI == U) {
-        Uses.push_back(&*AI);
-        Found = true;
-        break;
-      }
+    unsigned UseIndex =
+      std::distance(const_cast<const Use *>(CS->op_begin()), U);
+
+    assert(UseIndex < CS.arg_size() && "Non-argument use?");
+    if (UseIndex >= F->arg_size()) {
+      assert(F->isVarArg() && "More params than args in non-varargs call");
+      Captured = true;
+      return true;
     }
-    assert(Found && "Capturing call-site captured nothing?");
-    (void)Found;
+
+    auto FormalArgIt = F->arg_begin();
+    std::advance(FormalArgIt, UseIndex);
+    Uses.push_back(FormalArgIt);
     return false;
   }
 


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