SHUFP* are two address code.

[oota-llvm.git] / lib / Target / README.txt

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index d9091d0ca7698cb4e7ef3609d3898d2241d8cc6b..c2635dade4181062bd96f3476b220837dbc71afd 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -17,18 +17,14 @@ 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. :)
+Make the PPC branch selector target independant
 
-===-------------------------------------------------------------------------===
+//===---------------------------------------------------------------------===//
 
-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.
+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. :)
 
 //===---------------------------------------------------------------------===//
 
@@ -62,13 +58,83 @@ There are two ways to fix this:
 
 Number 1 is the preferred solution.
 
+This has been "fixed" by a TableGen hack. But that is a short term workaround
+which will be removed once the proper fix is made.
+
+//===---------------------------------------------------------------------===//
+
+Turn this into a signed shift right in instcombine:
+
+int f(unsigned x) {
+  return x >> 31 ? -1 : 0;
+}
+
+http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25600
+http://gcc.gnu.org/ml/gcc-patches/2006-02/msg01492.html
+
+//===---------------------------------------------------------------------===//
+
+On targets with expensive 64-bit multiply, we could LSR this:
+
+for (i = ...; ++i) {
+   x = 1ULL << i;
+
+into:
+ long long tmp = 1;
+ for (i = ...; ++i, tmp+=tmp)
+   x = tmp;
+
+This would be a win on ppc32, but not x86 or ppc64.
+
+//===---------------------------------------------------------------------===//
+
+Shrink: (setlt (loadi32 P), 0) -> (setlt (loadi8 Phi), 0)
+
+//===---------------------------------------------------------------------===//
+
+Reassociate should turn: X*X*X*X -> t=(X*X) (t*t) to eliminate a multiply.
+
+//===---------------------------------------------------------------------===//
+
+Interesting? testcase for add/shift/mul reassoc:
+
+int bar(int x, int y) {
+  return x*x*x+y+x*x*x*x*x*y*y*y*y;
+}
+int foo(int z, int n) {
+  return bar(z, n) + bar(2*z, 2*n);
+}
+
 //===---------------------------------------------------------------------===//
 
-For dag combiner and instcombine:
-Fold: "A / (B << N)" where B is a power of 2, to "A >> (N + log2(B))".
-Fold: "A % (B << N)" where B is a power of 2, to "A & ((B << N) - 1)".
+These two functions should generate the same code on big-endian systems:
 
-int t(int X, int Y) { return 1 << (X+4); }  --> 16 << X
+int g(int *j,int *l)  {  return memcmp(j,l,4);  }
+int h(int *j, int *l) {  return *j - *l; }
 
+this could be done in SelectionDAGISel.cpp, along with other special cases,
+for 1,2,4,8 bytes.
 
 //===---------------------------------------------------------------------===//
+
+This code:
+int rot(unsigned char b) { int a = ((b>>1) ^ (b<<7)) & 0xff; return a; }
+
+Can be improved in two ways:
+
+1. The instcombiner should eliminate the type conversions.
+2. The X86 backend should turn this into a rotate by one bit.
+
+//===---------------------------------------------------------------------===//
+
+Add LSR exit value substitution. It'll probably be a win for Ackermann, etc.
+
+//===---------------------------------------------------------------------===//
+
+It would be nice to revert this patch:
+http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20060213/031986.html
+
+And teach the dag combiner enough to simplify the code expanded before 
+legalize.  It seems plausible that this knowledge would let it simplify other
+stuff too.
+