Repro with:
$ llvm-g++ t.cc -O3 -S -o - -fno-exceptions -fomit-frame-pointer -m64

Here's a reduced testcase:

#include <string.h>
struct foo { int A, B; };
int test(struct foo *P) {
struct foo L;
memcpy(&L, P, sizeof(struct foo));
return L.A;
}

    %struct.foo = type { i32, i32 }

implementation ; Functions:

define i32 @​test(%struct.foo* %P) {
entry:
%L = alloca %struct.foo, align 8 ; <%struct.foo*> [#uses=2]
%L2 = bitcast %struct.foo* %L to i8* ; <i8*> [#uses=1]
%tmp13 = bitcast %struct.foo* %P to i8* ; <i8*> [#uses=1]
call void @​llvm.memcpy.i32( i8* %L2, i8* %tmp13, i32 8, i32 4 )
%tmp4 = getelementptr %struct.foo* %L, i32 0, i32 0 ; <i32*> [#uses=1]
%tmp5 = load i32* %tmp4 ; [#uses=1]
ret i32 %tmp5
}

This patch contains the (disabled) code to do the SROA. Before this can be enabled, mem2reg needs to be
able to promote scalars targetted by memcpy/memset etc.

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070305/045540.html

-Chris

Second half committed here:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070305/045718.html

Third 'half' will come later.

For my notes:

The main part of this is implemented, but the testcase in this bug is not yet implemented. The reason
for this is that the lowered structure starts out with an array of bytes, so we get a memcpy ("gep x, 0,
0") instead of memcpy(bitcast). Two issues:

1. Make xform work with GEP.
2. Figure out why we're getting the array of bytes in this trivial case.

Another testcase:

#include <string.h>

struct foo { int A, B; };

struct bar{ struct foo x; long long y; double D; };

int test1(struct foo *P) {
struct foo L;
memcpy(&L, P, sizeof(struct foo));
return L.A;
}

int test2() {
struct foo L[4];
memset(L, 0, sizeof(struct foo)*4);
return L[0].A;
}

int test3() {
struct bar B;
memset(&B, 1, sizeof(B));

B.x.A = 1;
B.D = 10.0;
return B.x.B;
}

-Chris

Here is the final piece:
http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20070312/045911.html

Testcases here:
Transforms/ScalarRepl/memset-aggregate.ll
Transforms/ScalarRepl/memset-aggregate-byte-leader.ll

-Chris

mentioned in issue #824