http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57690
Bug ID: 57690
Summary: bextr sometimes used instead of shr
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: jakub at gcc dot gnu.org
unsigned int bar (void);
unsigned long foo (unsigned int x) { return bar () >> 2; }
With -O2 -mtbm we get:
0: 48 83 ec 08 sub $0x8,%rsp
4: e8 00 00 00 00 callq 9 <foo+0x9>
5: R_X86_64_PC32 bar-0x4
9: 48 83 c4 08 add $0x8,%rsp
d: 8f ea f8 10 c0 02 1e bextr $0x1e02,%rax,%rax
14: 00 00
16: c3 retq
while without it:
0: 48 83 ec 08 sub $0x8,%rsp
4: e8 00 00 00 00 callq 9 <foo+0x9>
5: R_X86_64_PC32 bar-0x4
9: 48 83 c4 08 add $0x8,%rsp
d: c1 e8 02 shr $0x2,%eax
10: c3 retq
which is much shorter. On the other side, bextr with immediate gives more
freedom to the register allocator, because it is a non-destructive source
instruction. So, perhaps we want a peephole2 which will transform some forms
of the immediate TARGET_TBM tbm_bextr* (those where upper bits of a SImode or
DImode value are extracted and where destination is the same as source) into
shrl.
For -Os maybe it would be even shorter to emit movl + shrl.
