Put some of my random notes somewhere public

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@23897 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2005-10-23 19:52:42 +00:00
parent cc94b5178f
commit 1171ff4fd7

70
lib/Target/X86/README.txt Normal file
View File

@ -0,0 +1,70 @@
//===---------------------------------------------------------------------===//
// Random ideas for the X86 backend.
//===---------------------------------------------------------------------===//
Add a MUL2U and MUL2S nodes to represent a multiply that returns both the
Hi and Lo parts (combination of MUL and MULH[SU] into one node). Add this to
X86, & make the dag combiner produce it when needed. This will eliminate one
imul from the code generated for:
long long test(long long X, long long Y) { return X*Y; }
by using the EAX result from the mul. We should add a similar node for
DIVREM.
//===---------------------------------------------------------------------===//
This should be one DIV/IDIV instruction, not a libcall:
unsigned test(unsigned long long X, unsigned Y) {
return X/Y;
}
This can be done trivially with a custom legalizer. What about overflow
though? http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224
//===---------------------------------------------------------------------===//
Need to add support for rotate instructions.
//===---------------------------------------------------------------------===//
Some targets (e.g. athlons) prefer freep to fstp ST(0):
http://gcc.gnu.org/ml/gcc-patches/2004-04/msg00659.html
//===---------------------------------------------------------------------===//
This should use faddi on chips where it is profitable:
double foo(double P, int *I) { return P+*I; }
//===---------------------------------------------------------------------===//
The FP stackifier needs to be global. Also, it should handle simple permutates
to reduce number of shuffle instructions, e.g. turning:
fld P -> fld Q
fld Q fld P
fxch
or:
fxch -> fucomi
fucomi jl X
jg X
//===---------------------------------------------------------------------===//
Improvements to the multiply -> shift/add algorithm:
http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html
//===---------------------------------------------------------------------===//
Improve code like this (occurs fairly frequently, e.g. in LLVM):
long long foo(int x) { return 1LL << x; }
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html
Another useful one would be ~0ULL >> X and ~0ULL << X.