Random note about bswap.

llvm-svn: 108396

llvm/lib/Target/ARM/README.txt

@@ -631,3 +631,29 @@ The following is much shorter:
 
 
 //===---------------------------------------------------------------------===//
+
+The code generated for bswap on armv4/5 (CPUs without rev) is less than ideal:
+
+int a(int x) { return __builtin_bswap32(x); }
+
+a:
+	mov	r1, #255, 24
+	mov	r2, #255, 16
+	and	r1, r1, r0, lsr #8
+	and	r2, r2, r0, lsl #8
+	orr	r1, r1, r0, lsr #24
+	orr	r0, r2, r0, lsl #24
+	orr	r0, r0, r1
+	bx	lr
+
+Something like the following would be better (fewer instructions/registers):
+	eor     r1, r0, r0, ror #16
+	bic     r1, r1, #0xff0000
+	mov     r1, r1, lsr #8
+	eor     r0, r1, r0, ror #8
+	bx	lr
+
+A custom Thumb version would also be a slight improvement over the generic
+version.
+
+//===---------------------------------------------------------------------===//