[llvm-commits] [llvm] r47705 - /llvm/trunk/lib/Target/README.txt

[Chris Lattner]

Author: lattner
Date: Wed Feb 27 23:34:27 2008
New Revision: 47705

URL: http://llvm.org/viewvc/llvm-project?rev=47705&view=rev
Log:
target-indep codegen memcpy lowering issue.

Modified:
    llvm/trunk/lib/Target/README.txt

Modified: llvm/trunk/lib/Target/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/README.txt?rev=47705&r1=47704&r2=47705&view=diff

==============================================================================
--- llvm/trunk/lib/Target/README.txt (original)
+++ llvm/trunk/lib/Target/README.txt Wed Feb 27 23:34:27 2008
@@ -628,3 +628,47 @@
 
 We should add an FRINT node to the DAG to model targets that have legal
 implementations of ceil/floor/rint.
+
+//===---------------------------------------------------------------------===//
+
+Consider:
+
+int test() {
+  long long input[8] = {1,1,1,1,1,1,1,1};
+  foo(input);
+}
+
+We currently compile this into a memcpy from a global array since the 
+initializer is fairly large and not memset'able.  This is good, but the memcpy
+gets lowered to load/stores in the code generator.  This is also ok, except
+that the codegen lowering for memcpy doesn't handle the case when the source
+is a constant global.  This gives us atrocious code like this:
+
+	call	"L1$pb"
+"L1$pb":
+	popl	%eax
+	movl	_C.0.1444-"L1$pb"+32(%eax), %ecx
+	movl	%ecx, 40(%esp)
+	movl	_C.0.1444-"L1$pb"+20(%eax), %ecx
+	movl	%ecx, 28(%esp)
+	movl	_C.0.1444-"L1$pb"+36(%eax), %ecx
+	movl	%ecx, 44(%esp)
+	movl	_C.0.1444-"L1$pb"+44(%eax), %ecx
+	movl	%ecx, 52(%esp)
+	movl	_C.0.1444-"L1$pb"+40(%eax), %ecx
+	movl	%ecx, 48(%esp)
+	movl	_C.0.1444-"L1$pb"+12(%eax), %ecx
+	movl	%ecx, 20(%esp)
+	movl	_C.0.1444-"L1$pb"+4(%eax), %ecx
+...
+
+instead of:
+	movl	$1, 16(%esp)
+	movl	$0, 20(%esp)
+	movl	$1, 24(%esp)
+	movl	$0, 28(%esp)
+	movl	$1, 32(%esp)
+	movl	$0, 36(%esp)
+	...
+
+//===---------------------------------------------------------------------===//