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

[Hari Limaye via llvm-commits]

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@@ -716,10 +745,16 @@ 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.
+  if (isArgUnmodifiedByAllCalls(Arg, LocationSize::precise(Offset), FAM))
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hazzlim wrote:

Ah yes - I was mistakenly thinking that we bail when any load has a negative offset from Arg, but this is only the case for loads in the entry block that are guaranteed to execute. And I think what I've done here is in fact incorrect for all cases where the lowest offset is non-zero.

Given that it is not really possible to test this at this stage, I have reverted to not using any Size information if you think that seems appropriate?

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