Fix GetConstantStringInfo to not look into MDString (it works on

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

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index c788360df63f6a2de56c376b400f5ea078072c8f..e1772c2ead0eba78b6ee0cdebe3e3da636d0a2c5 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -801,8 +801,21 @@ void bar(unsigned n) {
     true();
 }
 
-I think this basically amounts to a dag combine to simplify comparisons against
-multiply hi's into a comparison against the mullo.
+This is equivalent to the following, where 2863311531 is the multiplicative
+inverse of 3, and 1431655766 is ((2^32)-1)/3+1:
+void bar(unsigned n) {
+  if (n * 2863311531U < 1431655766U)
+    true();
+}
+
+The same transformation can work with an even modulo with the addition of a
+rotate: rotate the result of the multiply to the right by the number of bits
+which need to be zero for the condition to be true, and shrink the compare RHS
+by the same amount.  Unless the target supports rotates, though, that
+transformation probably isn't worthwhile.
+
+The transformation can also easily be made to work with non-zero equality
+comparisons: just transform, for example, "n % 3 == 1" to "(n-1) % 3 == 0".
 
 //===---------------------------------------------------------------------===//
 
@@ -823,20 +836,6 @@ int main() {
 
 //===---------------------------------------------------------------------===//
 
-Instcombine will merge comparisons like (x >= 10) && (x < 20) by producing (x -
-10) u< 10, but only when the comparisons have matching sign.
-
-This could be converted with a similiar technique. (PR1941)
-
-define i1 @test(i8 %x) {
-  %A = icmp uge i8 %x, 5
-  %B = icmp slt i8 %x, 20
-  %C = and i1 %A, %B
-  ret i1 %C
-}
-
-//===---------------------------------------------------------------------===//
-
 These functions perform the same computation, but produce different assembly.
 
 define i8 @select(i8 %x) readnone nounwind {
@@ -884,18 +883,6 @@ The expression should optimize to something like
 
 //===---------------------------------------------------------------------===//
 
-From GCC Bug 15241:
-unsigned int
-foo (unsigned int a, unsigned int b)
-{
- if (a <= 7 && b <= 7)
-   baz ();
-}
-Should combine to "(a|b) <= 7".  Currently not optimized with "clang
--emit-llvm-bc | opt -std-compile-opts".
-
-//===---------------------------------------------------------------------===//
-
 From GCC Bug 3756:
 int
 pn (int n)
@@ -907,19 +894,6 @@ Should combine to (n >> 31) | 1.  Currently not optimized with "clang
 
 //===---------------------------------------------------------------------===//
 
-From GCC Bug 28685:
-int test(int a, int b)
-{
- int lt = a < b;
- int eq = a == b;
-
- return (lt || eq);
-}
-Should combine to "a <= b".  Currently not optimized with "clang
--emit-llvm-bc | opt -std-compile-opts | llc".
-
-//===---------------------------------------------------------------------===//
-
 void a(int variable)
 {
  if (variable == 4 || variable == 6)
@@ -993,12 +967,6 @@ Should combine to 0.  Currently not optimized with "clang
 
 //===---------------------------------------------------------------------===//
 
-int a(unsigned char* b) {return *b > 99;}
-There's an unnecessary zext in the generated code with "clang
--emit-llvm-bc | opt -std-compile-opts".
-
-//===---------------------------------------------------------------------===//
-
 int a(unsigned b) {return ((b << 31) | (b << 30)) >> 31;}
 Should be combined to  "((b >> 1) | b) & 1".  Currently not optimized
 with "clang -emit-llvm-bc | opt -std-compile-opts".
@@ -1011,12 +979,6 @@ Should combine to "x | (y & 3)".  Currently not optimized with "clang
 
 //===---------------------------------------------------------------------===//
 
-unsigned a(unsigned a) {return ((a | 1) & 3) | (a & -4);}
-Should combine to "a | 1".  Currently not optimized with "clang
--emit-llvm-bc | opt -std-compile-opts".
-
-//===---------------------------------------------------------------------===//
-
 int a(int a, int b, int c) {return (~a & c) | ((c|a) & b);}
 Should fold to "(~a & c) | (a & b)".  Currently not optimized with
 "clang -emit-llvm-bc | opt -std-compile-opts".