llvm-6502/lib/Target/README.txt
Chris Lattner 33872c99be This xform isn't safe
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@26007 91177308-0d34-0410-b5e6-96231b3b80d8
2006-02-05 08:26:16 +00:00

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Target Independent Opportunities:
===-------------------------------------------------------------------------===
FreeBench/mason contains code like this:
static p_type m0u(p_type p) {
int m[]={0, 8, 1, 2, 16, 5, 13, 7, 14, 9, 3, 4, 11, 12, 15, 10, 17, 6};
p_type pu;
pu.a = m[p.a];
pu.b = m[p.b];
pu.c = m[p.c];
return pu;
}
We currently compile this into a memcpy from a static array into 'm', then
a bunch of loads from m. It would be better to avoid the memcpy and just do
loads from the static array.
===-------------------------------------------------------------------------===
Get the C front-end to expand hypot(x,y) -> llvm.sqrt(x*x+y*y) when errno and
precision don't matter (ffastmath). Misc/mandel will like this. :)
===-------------------------------------------------------------------------===
For all targets, not just X86:
When llvm.memcpy, llvm.memset, or llvm.memmove are lowered, they should be
optimized to a few store instructions if the source is constant and the length
is smallish (< 8). This will greatly help some tests like Shootout/strcat.c
and fldry.
//===---------------------------------------------------------------------===//
Solve this DAG isel folding deficiency:
int X, Y;
void fn1(void)
{
X = X | (Y << 3);
}
compiles to
fn1:
movl Y, %eax
shll $3, %eax
orl X, %eax
movl %eax, X
ret
The problem is the store's chain operand is not the load X but rather
a TokenFactor of the load X and load Y, which prevents the folding.
There are two ways to fix this:
1. The dag combiner can start using alias analysis to realize that y/x
don't alias, making the store to X not dependent on the load from Y.
2. The generated isel could be made smarter in the case it can't
disambiguate the pointers.
Number 1 is the preferred solution.
//===---------------------------------------------------------------------===//