[llvm] [ArgPromotion] Perform alias analysis on actual arguments of Calls (PR #106216)

[Eli Friedman via llvm-commits]

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@@ -716,10 +744,22 @@ static bool findArgParts(Argument *Arg, const DataLayout &DL, AAResults &AAR,
     return true;
 
   // Okay, now we know that the argument is only used by load instructions, and
-  // it is safe to unconditionally perform all of them. Use alias analysis to
-  // check to see if the pointer is guaranteed to not be modified from entry of
-  // the function to each of the load instructions.
+  // it is safe to unconditionally perform all of them.
+
+  // If we can determine that no call to the Function modifies the memory region
+  // accessed through Arg, through alias analysis using actual arguments in the
+  // callers, we know that it is guaranteed to be safe to promote the argument.
+
+  // Compute the size of the memory region accessed by the Loads through Arg.
+  LocationSize Size = LocationSize::precise(0);
+  for (LoadInst *Load : Loads) {
+    Size = Size.unionWith(MemoryLocation::get(Load).Size);
+  }
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efriedma-quic wrote:

This loop doesn't compute what the comment says it does.  Say you have an array of 3 uint32_t to promote; that's 12 bytes.  This computes a size of 4 bytes.

https://github.com/llvm/llvm-project/pull/106216