[PATCH] D21246: [InstCombine] Add rule to fold away trunc of partial load

[Anna Thomas via llvm-commits]

anna added inline comments.

================
Comment at: lib/Analysis/Loads.cpp:385
@@ +384,3 @@
+      // If we support partial value forwarding to the load (PartialType is
+      // non-null): LI is from a part of the bits (known from PartialType)
+      // used by Load, return the value for those bits. The PartialType is from
----------------
sanjoy wrote:
> It feels odd that we have to special-case `PartialType` this way.  I think a better design would be to introduce a new function `FindAvailableMemoryContents(Value *Ptr, Type *AccessTy ... < no PartialTy>)` (perhaps with a different name), and have this version of `FindAvailableLoadedValue` dispatch to `FindAvailableMemoryContents` one with `LI->getPointerOperand(), LI->getType()`.  `visitTrunc` can then call `FindAvailableMemoryContents` directly.
It seems ok, but are we fine with changing `FindAvailableLoadedValue` to being a wrapper function, with all the actual code in `FindAvailableMemoryContents`?

================
Comment at: lib/Transforms/InstCombine/InstCombineCasts.cpp:588
@@ +587,3 @@
+    if (Value *AvailableVal = FindAvailableLoadedValue(
+            LI, LI->getParent(), BBI, DefMaxInstsToScan,
+            /* AA */ nullptr, /* AATags */ nullptr, DestTy))
----------------
sanjoy wrote:
> Here if we have:
> 
> ```
> store i64 %x to %ptr
> %val = load i64* %ptr
> %val.tr = trunc i64 %val to i32
> ```
> 
> won't we have a type mismatch and try to replace `%val.tr` with `%x`?
We won't replace `%val.tr` with `%x` since the `DestTy` is `i32`. We do the check in  `FindAvailableLoadedValue (LI: %val,.., DestTy: i32)` that `i64` is `BitorNoopPointerCastable` to the PartialType `i32`. 
I've added this test case as well now. 


http://reviews.llvm.org/D21246