Code cleanup. Provide generic way of selecting JIT pointer bitwidth regardless

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

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 80b00377ae71486baa1820baac2e001be1a3f1f9..c64d7e0e6e61d9093f667aabd9aa5617f9603c48 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -705,3 +705,78 @@ int f() {
 }
 
 //===---------------------------------------------------------------------===//
+
+The loop unroller should partially unroll loops (instead of peeling them)
+when code growth isn't too bad and when an unroll count allows simplification
+of some code within the loop.  One trivial example is:
+
+#include <stdio.h>
+int main() {
+    int nRet = 17;
+    int nLoop;
+    for ( nLoop = 0; nLoop < 1000; nLoop++ ) {
+        if ( nLoop & 1 )
+            nRet += 2;
+        else
+            nRet -= 1;
+    }
+    return nRet;
+}
+
+Unrolling by 2 would eliminate the '&1' in both copies, leading to a net
+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.
+
+//===---------------------------------------------------------------------===//
+