Just so this doesn't get lost, I'm filing a "wishlist" item for this.

Date: Wed, 6 Oct 2004 14:25:30 -0500 (CDT)
From: Chris Lattner <sabre@nondot.org>
Subject: memset optimizations

memset currently inhibits multiple optimizations, e.g. memset/load doesn't
get copy-propagated, memset doesn't cause dead stores to die, and memset
inhibits SRA.  This should be fixed!

Also: memset(&X, 4) -> store int 0, X   iff X is aligned

Also, memset should not kill unaliased loads/stores.

Created an attachment (id=759) [details]
Edits a hard-coded string in an unrolled loop

I was told in the IRC channel that the non optimized LLVM code I was getting of
the attached C++ code was related to this issue.
It becomes optimized when the string 's' becomes small, like "this".

Bug 1309 is also related to this.

Created an attachment (id=1358) [details]
example with poor memcpy optimization

In the last line
        return r + x.A;
the value of x.A is zero, but LLVM doesn't understand this.
Presumably this is because it thinks memcpy has modified x,
even though it copies from x and not to it.

Bug 1952 is one specific optimization that we can do, relating it.

This has gotten a lot better recently, with work Chris and I have done which is
now in the MemCpyOpt pass.  There's still a lot to do, though.