[Patch] Enable value forwarding for loads from calloc without intervening store

[Philip Reames]

(I have no idea why this didn't get sent to the mailing list, but here's 
the manual forward...)

Enable value forwarding for loads from calloc without intervening store
http://reviews.llvm.org/D3931

This change extends GVN to handle the following case:
%1 = tail call noalias i8* @calloc(i64 1, i64 4) #2
%2 = bitcast i8* %1 to i32*
; This load is trivially constant zero
%3 = load i32* %2, align 4

This is analogous to the handling for malloc in the same places. Malloc 
returns undef, calloc returns a zero value. Note that it is correct to 
return zero even for out of bounds geps since the result of such a gep 
would be undefined.

I'm not entirely clear why this code lives in GVN rather than elsewhere; 
I have chosen to simply extend the existing design.

Please note: I do not have commit access and will need someone to commit 
this on my behalf.

Philip
-------------- next part --------------
A non-text attachment was scrubbed...
Name: calloc-load-forward.diff
Type: text/x-patch
Size: 2159 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20140528/ddff20e9/attachment.bin>