Support for expanding the result of EXTRACT_ELEMENT.

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

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index d477c82fbb260421b368b4d1952b13c7ae4a59e7..23572cd6bc58f1f0bda5cd7be10e25da07c8b50a 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -317,11 +317,6 @@ this construct.
 
 //===---------------------------------------------------------------------===//
 
-Instcombine misses several of these cases (see the testcase in the patch):
-http://gcc.gnu.org/ml/gcc-patches/2006-10/msg01519.html
-
-//===---------------------------------------------------------------------===//
-
 viterbi speeds up *significantly* if the various "history" related copy loops
 are turned into memcpy calls at the source level.  We need a "loops to memcpy"
 pass.
@@ -728,3 +723,64 @@ reduction in code size.  The resultant code would then also be suitable for
 exit value computation.
 
 //===---------------------------------------------------------------------===//
+
+We miss a bunch of rotate opportunities on various targets, including ppc, x86,
+etc.  On X86, we miss a bunch of 'rotate by variable' cases because the rotate
+matching code in dag combine doesn't look through truncates aggressively 
+enough.  Here are some testcases reduces from GCC PR17886:
+
+unsigned long long f(unsigned long long x, int y) {
+  return (x << y) | (x >> 64-y); 
+} 
+unsigned f2(unsigned x, int y){
+  return (x << y) | (x >> 32-y); 
+} 
+unsigned long long f3(unsigned long long x){
+  int y = 9;
+  return (x << y) | (x >> 64-y); 
+} 
+unsigned f4(unsigned x){
+  int y = 10;
+  return (x << y) | (x >> 32-y); 
+}
+unsigned long long f5(unsigned long long x, unsigned long long y) {
+  return (x << 8) | ((y >> 48) & 0xffull);
+}
+unsigned long long f6(unsigned long long x, unsigned long long y, int z) {
+  switch(z) {
+  case 1:
+    return (x << 8) | ((y >> 48) & 0xffull);
+  case 2:
+    return (x << 16) | ((y >> 40) & 0xffffull);
+  case 3:
+    return (x << 24) | ((y >> 32) & 0xffffffull);
+  case 4:
+    return (x << 32) | ((y >> 24) & 0xffffffffull);
+  default:
+    return (x << 40) | ((y >> 16) & 0xffffffffffull);
+  }
+}
+
+On X86-64, we only handle f3/f4 right.  On x86-32, several of these 
+generate truly horrible code, instead of using shld and friends.  On
+ARM, we end up with calls to L___lshrdi3/L___ashldi3 in f, which is
+badness.  PPC64 misses f, f5 and f6.  CellSPU aborts in isel.
+
+//===---------------------------------------------------------------------===//
+
+We do a number of simplifications in simplify libcalls to strength reduce
+standard library functions, but we don't currently merge them together.  For
+example, it is useful to merge memcpy(a,b,strlen(b)) -> strcpy.  This can only
+be done safely if "b" isn't modified between the strlen and memcpy of course.
+
+//===---------------------------------------------------------------------===//
+
+We should be able to evaluate this loop:
+
+int test(int x_offs) {
+  while (x_offs > 4)
+     x_offs -= 4;
+  return x_offs;
+}
+
+//===---------------------------------------------------------------------===//