http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60412
Bug ID: 60412
Summary: superfluous arithmetic generated for uneven tail
handling
Product: gcc
Version: 4.8.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jens.maurer at gmx dot net
Created attachment 32262
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=32262&action=edit
Testcase showing superfluous arithmetic in assembly
The attached function processes a (possibly long) memory range. If more than 7
bytes are available, those are processed using g1(), which is able to process 8
bytes in one call. The tail end is then handled by g2(), one byte at a time.
(In my application, the g1 and g2 functions are the Intel built-in CRC-32C
operations.)
On Intel x86-64, the following superfluous assembly code is produced between
the calls to g1() and g2():
$ gcc -v -O3 -Wall -Wextra -c opt-tail.c
...
Target: x86_64-unknown-linux-gnu
...
GNU C (GCC) version 4.8.2 (x86_64-unknown-linux-gnu)
compiled by GNU C version 4.8.2, GMP version 5.1.2, MPFR version
3.1.1-p2, MPC version 1.0.1
$ objdump -C -D opt-tail.o
...
44: 4c 89 e2 mov %r12,%rdx
47: 48 29 da sub %rbx,%rdx
4a: 48 83 ea 08 sub $0x8,%rdx
4e: 48 c1 ea 03 shr $0x3,%rdx
52: 48 8d 5c d3 08 lea 0x8(%rbx,%rdx,8),%rbx
This seems to be completely redundant.
