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target/i386: use TSTEQ/TSTNE to test low bits 

When testing the sign bit or equality to zero of a partial register, it
is useful to use a single TSTEQ or TSTNE operation.  It can also be used
to test the parity flag, using bit 0 of the population count.

Do not do this for target_ulong-sized values however; the optimizer would
produce a comparison against zero anyway, and it avoids shifts by 64
which are undefined behavior.

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 05:57:31 +02:00
parent b776569a53
commit 15957eb9ef
2 changed files with 22 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

5
target/i386/tcg/emit.c.inc
View File

@ -1209,7 +1209,7 @@ static void gen_CMPccXADD(DisasContext *s, CPUX86State *env, X86DecodedInsn *dec
[JCC_Z] = TCG_COND_EQ,
[JCC_BE] = TCG_COND_LEU,
[JCC_S] = TCG_COND_LT, /* test sign bit by comparing against 0 */
[JCC_P] = TCG_COND_EQ, /* even parity - tests low bit of popcount */
[JCC_P] = TCG_COND_TSTEQ, /* even parity - tests low bit of popcount */
[JCC_L] = TCG_COND_LT,
[JCC_LE] = TCG_COND_LE,
};
@ -1260,8 +1260,7 @@ static void gen_CMPccXADD(DisasContext *s, CPUX86State *env, X86DecodedInsn *dec
case JCC_P:
tcg_gen_ext8u_tl(s->tmp0, s->T0);
tcg_gen_ctpop_tl(s->tmp0, s->tmp0);
tcg_gen_andi_tl(s->tmp0, s->tmp0, 1);
cmp_lhs = s->tmp0, cmp_rhs = tcg_constant_tl(0);
cmp_lhs = s->tmp0, cmp_rhs = tcg_constant_tl(1);
break;
case JCC_S:

28
target/i386/tcg/translate.c
View File

@ -928,11 +928,21 @@ typedef struct CCPrepare {
bool no_setcond;
} CCPrepare;
static CCPrepare gen_prepare_sign_nz(TCGv src, MemOp size)
{
if (size == MO_TL) {
return (CCPrepare) { .cond = TCG_COND_LT, .reg = src, .mask = -1 };
} else {
return (CCPrepare) { .cond = TCG_COND_TSTNE, .reg = src, .mask = -1,
.imm = 1ull << ((8 << size) - 1) };
}
}
/* compute eflags.C to reg */
static CCPrepare gen_prepare_eflags_c(DisasContext *s, TCGv reg)
{
TCGv t0, t1;
int size, shift;
MemOp size;
switch (s->cc_op) {
case CC_OP_SUBB ... CC_OP_SUBQ:
@ -967,9 +977,7 @@ static CCPrepare gen_prepare_eflags_c(DisasContext *s, TCGv reg)
case CC_OP_SHLB ... CC_OP_SHLQ:
/* (CC_SRC >> (DATA_BITS - 1)) & 1 */
size = s->cc_op - CC_OP_SHLB;
shift = (8 << size) - 1;
return (CCPrepare) { .cond = TCG_COND_NE, .reg = cpu_cc_src,
.mask = (target_ulong)1 << shift };
return gen_prepare_sign_nz(cpu_cc_src, size);
case CC_OP_MULB ... CC_OP_MULQ:
return (CCPrepare) { .cond = TCG_COND_NE,
@ -1029,8 +1037,7 @@ static CCPrepare gen_prepare_eflags_s(DisasContext *s, TCGv reg)
default:
{
MemOp size = (s->cc_op - CC_OP_ADDB) & 3;
TCGv t0 = gen_ext_tl(reg, cpu_cc_dst, size, true);
return (CCPrepare) { .cond = TCG_COND_LT, .reg = t0, .mask = -1 };
return gen_prepare_sign_nz(cpu_cc_dst, size);
}
}
}
@ -1077,8 +1084,13 @@ static CCPrepare gen_prepare_eflags_z(DisasContext *s, TCGv reg)
default:
{
MemOp size = (s->cc_op - CC_OP_ADDB) & 3;
TCGv t0 = gen_ext_tl(reg, cpu_cc_dst, size, false);
return (CCPrepare) { .cond = TCG_COND_EQ, .reg = t0, .mask = -1 };
if (size == MO_TL) {
return (CCPrepare) { .cond = TCG_COND_EQ, .reg = cpu_cc_dst,
.mask = -1 };
} else {
return (CCPrepare) { .cond = TCG_COND_TSTEQ, .reg = cpu_cc_dst,
.mask = -1, .imm = (1ull << (8 << size)) - 1 };
}
}
}
}