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target/i386: optimize CX handling in repeated string operations

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In a repeated string operation, CX/ECX will be decremented until it
is 0 but never underflow.  Use this observation to avoid a deposit or
zero-extend operation if the address size of the operation is smaller
than MO_TL.

As in the previous patch, the patch is structured to include some
preparatory work for subsequent changes.  In particular, introducing
cx_next prepares for when ECX will be decremented *before* calling
fn(s, ot), and therefore cannot yet be written back to cpu_regs.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Link: https://lore.kernel.org/r/20241215090613.89588-11-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2024-12-15 10:06:09 +01:00
parent 3658116025
commit 0360b78187
1 changed files with 14 additions and 1 deletions
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15
target/i386/tcg/translate.c
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@ -1339,6 +1339,7 @@ static void do_gen_rep(DisasContext *s, MemOp ot,
{
TCGLabel *done = gen_new_label();
target_ulong cx_mask = MAKE_64BIT_MASK(0, 8 << s->aflag);
TCGv cx_next = tcg_temp_new();
bool had_rf = s->flags & HF_RF_MASK;
/*
@ -1364,7 +1365,19 @@ static void do_gen_rep(DisasContext *s, MemOp ot,
tcg_gen_brcondi_tl(TCG_COND_TSTEQ, cpu_regs[R_ECX], cx_mask, done);
fn(s, ot);
gen_op_add_reg_im(s, s->aflag, R_ECX, -1);
tcg_gen_subi_tl(cx_next, cpu_regs[R_ECX], 1);
/*
* Write back cx_next to CX/ECX/RCX. There can be no carry, so zero
* extend if needed but do not do expensive deposit operations.
*/
#ifdef TARGET_X86_64
if (s->aflag == MO_32) {
tcg_gen_ext32u_tl(cx_next, cx_next);
}
#endif
tcg_gen_mov_tl(cpu_regs[R_ECX], cx_next);
gen_update_cc_op(s);
/* Leave if REP condition fails. */