[PATCH] D27853: [CodeGenPrep]Restructure promoting Ext to form ExtLoad

[Jun Bum Lim via Phabricator via llvm-commits]

junbuml added inline comments.


================
Comment at: lib/CodeGen/CodeGenPrepare.cpp:4305
     TotalCreatedInstsCost -= ExtCost;
+    if (TotalCreatedInstsCost < 0)
+      TotalCreatedInstsCost = 0;
----------------
mcrosier wrote:
> You might also write this as
> 
>   TotalCreatedInstsCost = std::max(0, TotalCreatedInstsCost - ExtCost);
> 
> Can you please add a comment as to why we ensure TotalCreatedInstsCost is non-negative?  I believe it's because we're passing a signed value to tryToPromoteExts(), which expects an unsigned argument.
> 
> Lastly, is this something that could be fixed independently of this patch?  And do you have a test case that covers this change?
Yes, the signed value TotalCreatedInstsCost is passed as unsigned argument,  the negative value of TotalCreatedInstsCost could mislead the cost model.   

Let me separate this as an independent patch with a test case. 


https://reviews.llvm.org/D27853