Clang initialization code generation can be improved

[lattner]

Bugzilla Link	279
Resolution	FIXED
Resolved on	Dec 02, 2010 01:07
Version	unspecified
OS	All
CC	@lattner

Extended Description

Though we are generating better code for GCC CONSTRUCTOR nodes (See Bug 275), we
still aren't doing as well as we should do. In particular, we handle cases like
this poorly:

int foo(int X) {
int Arr[100] = { X }; // Should use memset
int Arr[10000] = { 1 }; // Should use memset

// Should memcpy the whole thing
struct { int A, B } Pairs[10] = { {1, 2 }, { 0, 0 } };
}

... While the first ones could be addressed right now, the last one requires a
rewrite of how we handle constructors, which is needed to fix several code
correctness bugs anyway. This bug is just a tracker for when this happens.

-Chris

[lattner]

Here's a testcase in the regression suite:
test/Regression/CFrontend/2004-02-13-StringInit.c.tr

[lattner]

Here's another testcase:

struct T { int A, B; };
struct S { struct T X, Y; };

int test1(int x) {
struct S A = { {x, x+1}, {0, 12 } };
}

int test2() {
struct S A[] = {
{ { 0, 1}, {2, 3} },
{ { 2, 1}, {2, 3} },
{ { 3, 1}, {2, 3} },
{ { 4, 1}, {2, 3} },
{ { 5, 1}, {2, 3} },
};
}

int test3(int x) {
const int array[] = {
17, x, 123, 123, 49, 17, 23,
};
return array[x];
}

[lattner]

Here is the example from comment #​1:

// RUN: %llvmgcc -xc %s -c -o - | llvm-dis | grep llvm.memset
char test(int X) {
char str[10000] = "abc"; // tail should be memset.
return str[X];
}

[llvmbot]

From comment #​1, we memset the first two arrays. However, we use a bunch of SSE instructions to initialize the "Pairs" variable:

xorps	%xmm0, %xmm0
movaps	%xmm0, -24(%ebp)
movaps	%xmm0, -40(%ebp)
movaps	%xmm0, -56(%ebp)
movaps	%xmm0, -72(%ebp)
movaps	%xmm0, -88(%ebp)
movl	$1, -88(%ebp)
movl	$2, -84(%ebp)

[lattner]

We don't care about llvm-gcc anymore, so stealing this for Clang. After r120645, clang now generates the right code for the second Arr testcase in Comment 0, and also handles hte Pairs example nicely from Comment #​0. It handles the examples in comment #​2, and the "str" case from comment 3. The example in comment 1 doesn't exist anymore.

The one remaining issue is that it does not handle this well from comment #​0:

int foo(int X) {
int Arr[100] = { X }; // Should use memset
}

[lattner]

Finished off in r120692. We now generate a memset + 1 store.