Add some potentially interesting transformations to README.

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

diff --git a/lib/Target/README.txt b/lib/Target/README.txt
index 38c3daa9383845a6d4884b91bc703eb3dff7556a..080ea42dccdd956b4e7e1fdac528bb7ac21ca236 100644 (file)
--- a/lib/Target/README.txt
+++ b/lib/Target/README.txt
@@ -1648,36 +1648,7 @@ would delete the or instruction for us.
 
 //===---------------------------------------------------------------------===//
 
-FunctionAttrs is not marking this function as readnone (just readonly):
-$ clang t.c -emit-llvm -S -o - -O0 | opt -mem2reg -S -functionattrs
-
-int t(int a, int b, int c) {
- int *p;
- if (a)
-   p = &a;
- else
-   p = &c;
- return *p;
-}
-
-This is because we codegen this to:
-
-define i32 @t(i32 %a, i32 %b, i32 %c) nounwind readonly ssp {
-entry:
-  %a.addr = alloca i32                            ; <i32*> [#uses=3]
-  %c.addr = alloca i32                            ; <i32*> [#uses=2]
-...
-
-if.end:
-  %p.0 = phi i32* [ %a.addr, %if.then ], [ %c.addr, %if.else ]
-  %tmp2 = load i32* %p.0                          ; <i32> [#uses=1]
-  ret i32 %tmp2
-}
-
-And functionattrs doesn't realize that the p.0 load points to function local
-memory.
-
-Also, functionattrs doesn't know about memcpy/memset.  This function should be
+functionattrs doesn't know much about memcpy/memset.  This function should be
 marked readnone rather than readonly, since it only twiddles local memory, but
 functionattrs doesn't handle memset/memcpy/memmove aggressively:
 
@@ -1694,3 +1665,54 @@ int foo() {
 
 //===---------------------------------------------------------------------===//
 
+Missed instcombine transformation:
+define i1 @a(i32 %x) nounwind readnone {
+entry:
+  %cmp = icmp eq i32 %x, 30
+  %sub = add i32 %x, -30
+  %cmp2 = icmp ugt i32 %sub, 9
+  %or = or i1 %cmp, %cmp2
+  ret i1 %or
+}
+This should be optimized to a single compare.  Testcase derived from gcc.
+
+//===---------------------------------------------------------------------===//
+
+Missed instcombine transformation:
+void b();
+void a(int x) { if (((1<<x)&8)==0) b(); }
+
+The shift should be optimized out.  Testcase derived from gcc.
+
+//===---------------------------------------------------------------------===//
+
+Missed instcombine or reassociate transformation:
+int a(int a, int b) { return (a==12)&(b>47)&(b<58); }
+
+The sgt and slt should be combined into a single comparison. Testcase derived
+from gcc.
+
+//===---------------------------------------------------------------------===//
+
+Missed instcombine transformation:
+define i32 @a(i32 %x) nounwind readnone {
+entry:
+  %shr = lshr i32 %x, 5                           ; <i32> [#uses=1]
+  %xor = xor i32 %shr, 67108864                   ; <i32> [#uses=1]
+  %sub = add i32 %xor, -67108864                  ; <i32> [#uses=1]
+  ret i32 %sub
+}
+
+This function is equivalent to "ashr i32 %x, 5".  Testcase derived from gcc.
+
+//===---------------------------------------------------------------------===//
+
+isSafeToLoadUnconditionally should allow a GEP of a global/alloca with constant
+indicies within the bounds of the allocated object. Reduced example:
+
+const int a[] = {3,6};
+int b(int y) { int* x = y ? &a[0] : &a[1]; return *x; }
+
+All the loads should be eliminated.  Testcase derived from gcc.
+
+//===---------------------------------------------------------------------===//