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| author | Richard Henderson <rth@twiddle.net> | 2013-01-23 15:43:03 -0800 |
|---|---|---|
| committer | Richard Henderson <rth@twiddle.net> | 2013-02-18 15:03:58 -0800 |
| commit | a3251186fc6a04d421e9c4b65aa04ec32379ec38 (patch) | |
| tree | 799b652f21822a322cf09912cb2da2ec50346662 /exec.c | |
| parent | 891a5133f1637296c3823229180b5851132ed5f5 (diff) | |
target-i386: optimize flags checking after sub using CC_SRCT
After a comparison or subtraction, the original value of the LHS will
currently be reconstructed using an addition. However, in most cases
it is already available: store it in a temp-local variable and save 1
or 2 TCG ops (2 if the result of the addition needs to be extended).
The temp-local can be declared dead as soon as the cc_op changes again,
or also before the translation block ends because gen_prepare_cc will
always make a copy before returning it. All this magic, plus copy
propagation and dead-code elimination, ensures that the temp local will
(almost) never be spilled.
Example (cmp $0x21,%rax + jbe):
Before After
----------------------------------------------------------------------------
movi_i64 tmp1,$0x21 movi_i64 tmp1,$0x21
movi_i64 cc_src,$0x21 movi_i64 cc_src,$0x21
sub_i64 cc_dst,rax,tmp1 sub_i64 cc_dst,rax,tmp1
add_i64 tmp7,cc_dst,cc_src
movi_i32 cc_op,$0x11 movi_i32 cc_op,$0x11
brcond_i64 tmp7,cc_src,leu,$0x0 discard loc11
brcond_i64 rax,cc_src,leu,$0x0
Before After
----------------------------------------------------------------------------
mov (%r14),%rbp mov (%r14),%rbp
mov %rbp,%rbx mov %rbp,%rbx
sub $0x21,%rbx sub $0x21,%rbx
lea 0x21(%rbx),%r12
movl $0x11,0xa0(%r14) movl $0x11,0xa0(%r14)
movq $0x21,0x90(%r14) movq $0x21,0x90(%r14)
mov %rbx,0x98(%r14) mov %rbx,0x98(%r14)
cmp $0x21,%r12 | cmp $0x21,%rbp
jbe ... jbe ...
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Richard Henderson <rth@twiddle.net>
Diffstat (limited to 'exec.c')
0 files changed, 0 insertions, 0 deletions