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: NPaolo Bonzini <pbonzini@redhat.com> Signed-off-by: NRichard Henderson <rth@twiddle.net>
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