6502/llvm-6502
1
0
Fork 0
mirror of https://github.com/c64scene-ar/llvm-6502.git synced 2025-01-14 00:32:55 +00:00
Code Issues Projects Releases Wiki Activity

memcpy + metadata = bliss :)

Browse Source 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123405 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-01-13 22:08:15 +00:00
parent e4e5e2aae7
commit 4cd18f98f7
1 changed files with 48 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

48
lib/Target/README.txt
View File

@ -2259,3 +2259,51 @@ Since we know that x+2.0 doesn't care about the sign of any zeros in X, we can
transform the fmul to 0.0, and then the fadd to 2.0.
//===---------------------------------------------------------------------===//
We should enhance memcpy/memcpy/memset to allow a metadata node on them
indicating that some bytes of the transfer are undefined. This is useful for
frontends like clang when lowering struct lowering, when some elements of the
struct are undefined. Consider something like this:
struct x {
char a;
int b[4];
};
void foo(struct x*P);
struct x testfunc() {
struct x V1, V2;
foo(&V1);
V2 = V1;
return V2;
}
We currently compile this to:
$ clang t.c -S -o - -O0 -emit-llvm | opt -scalarrepl -S
%struct.x = type { i8, [4 x i32] }
define void @testfunc(%struct.x* sret %agg.result) nounwind ssp {
entry:
%V1 = alloca %struct.x, align 4
call void @foo(%struct.x* %V1)
%tmp1 = bitcast %struct.x* %V1 to i8*
%0 = bitcast %struct.x* %V1 to i160*
%srcval1 = load i160* %0, align 4
%tmp2 = bitcast %struct.x* %agg.result to i8*
%1 = bitcast %struct.x* %agg.result to i160*
store i160 %srcval1, i160* %1, align 4
ret void
}
This happens because SRoA sees that the temp alloca has is being memcpy'd into
and out of and it has holes and it has to be conservative. If we knew about the
holes, then this could be much much better.
Having information about these holes would also improve memcpy (etc) lowering at
llc time when it gets inlined, because we can use smaller transfers. This also
avoids partial register stalls in some important cases.
//===---------------------------------------------------------------------===//