[llvm-commits] [llvm] r108396 - /llvm/trunk/lib/Target/ARM/README.txt

[Eli Friedman]

Author: efriedma
Date: Wed Jul 14 21:20:38 2010
New Revision: 108396

URL: http://llvm.org/viewvc/llvm-project?rev=108396&view=rev
Log:
Random note about bswap.


Modified:
    llvm/trunk/lib/Target/ARM/README.txt

Modified: llvm/trunk/lib/Target/ARM/README.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/ARM/README.txt?rev=108396&r1=108395&r2=108396&view=diff
==============================================================================
--- llvm/trunk/lib/Target/ARM/README.txt (original)
+++ llvm/trunk/lib/Target/ARM/README.txt Wed Jul 14 21:20:38 2010
@@ -631,3 +631,29 @@
 
 
 //===---------------------------------------------------------------------===//
+
+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.
+
+//===---------------------------------------------------------------------===//