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target/i386: optimize computation of JL and JLE from flags 

Take advantage of the fact that there can be no 1 bits between SF and OF.
If they were adjacent, you could sum SF and get a carry only if SF was
already set.  Then the value of OF in the sum is the XOR of OF itself,
the carry (which is SF) and 0 (the value of the OF bit in the addend):
this is OF^SF exactly.

Because OF and SF are not adjacent, just place more 1 bits to the
left so that the carry propagates, which means summing CC_O - CC_S.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2023-10-27 04:12:59 +02:00
parent 007531586a
commit 6032627f07
1 changed files with 4 additions and 6 deletions
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10
target/i386/tcg/translate.c
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@ -1126,10 +1126,9 @@ static CCPrepare gen_prepare_cc(DisasContext *s, int b, TCGv reg)
if (reg == cpu_cc_src) {
reg = s->tmp0;
}
tcg_gen_shri_tl(reg, cpu_cc_src, 4); /* CC_O -> CC_S */
tcg_gen_xor_tl(reg, reg, cpu_cc_src);
tcg_gen_addi_tl(reg, cpu_cc_src, CC_O - CC_S);
cc = (CCPrepare) { .cond = TCG_COND_NE, .reg = reg,
.mask = CC_S };
.mask = CC_O };
break;
default:
case JCC_LE:
@ -1137,10 +1136,9 @@ static CCPrepare gen_prepare_cc(DisasContext *s, int b, TCGv reg)
if (reg == cpu_cc_src) {
reg = s->tmp0;
}
tcg_gen_shri_tl(reg, cpu_cc_src, 4); /* CC_O -> CC_S */
tcg_gen_xor_tl(reg, reg, cpu_cc_src);
tcg_gen_addi_tl(reg, cpu_cc_src, CC_O - CC_S);
cc = (CCPrepare) { .cond = TCG_COND_NE, .reg = reg,
.mask = CC_S | CC_Z };
.mask = CC_O | CC_Z };
break;
}
break;