diff options
| -rw-r--r-- | llvm/lib/Target/ARM/README.txt | 26 |
1 files changed, 26 insertions, 0 deletions
diff --git a/llvm/lib/Target/ARM/README.txt b/llvm/lib/Target/ARM/README.txt index 83c4f67bd79..0cb8ff01181 100644 --- a/llvm/lib/Target/ARM/README.txt +++ b/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. + +//===---------------------------------------------------------------------===// |
