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Random note about bswap.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@108396 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eli Friedman 2010-07-15 02:20:38 +00:00
parent b5b73c4c86
commit 4baa8ebc93
1 changed files with 26 additions and 0 deletions
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26
lib/Target/ARM/README.txt
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@ -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.
//===---------------------------------------------------------------------===//