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tcg/optimize: optimize TSTNE using smask and zmask

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Generalize the existing optimization of "TSTNE x,sign" and "TSTNE x,-1".
This can be useful for example in the i386 frontend, which will generate
tests of zero-extended registers against 0xffffffff.

Ironically, on x86 hosts this is a very slight pessimization in the very
case it's meant to optimize because

 brcond_i64 cc_dst,$0xffffffff,tsteq,$L1

(test %ebx, %ebx) is 1 byte smaller than

 brcond_i64 cc_dst,$0x0,eq,$L1

(test %rbx, %rbx).  However, in general it is an improvement, especially
if it avoids placing a large immediate in the constant pool.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2024-01-22 10:48:11 +01:00
parent 059fad25fd
commit 3502062991
1 changed files with 8 additions and 5 deletions
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13
tcg/optimize.c
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@ -766,6 +766,7 @@ static int do_constant_folding_cond1(OptContext *ctx, TCGOp *op, TCGArg dest,
TCGArg *p1, TCGArg *p2, TCGArg *pcond)
{
TCGCond cond;
TempOptInfo *i1;
bool swap;
int r;
@ -783,19 +784,21 @@ static int do_constant_folding_cond1(OptContext *ctx, TCGOp *op, TCGArg dest,
return -1;
}
i1 = arg_info(*p1);
/*
* TSTNE x,x -> NE x,0
* TSTNE x,-1 -> NE x,0
* TSTNE x,i -> NE x,0 if i includes all nonzero bits of x
*/
if (args_are_copies(*p1, *p2) || arg_is_const_val(*p2, -1)) {
if (args_are_copies(*p1, *p2) ||
(arg_is_const(*p2) && (i1->z_mask & ~arg_info(*p2)->val) == 0)) {
*p2 = arg_new_constant(ctx, 0);
*pcond = tcg_tst_eqne_cond(cond);
return -1;
}
/* TSTNE x,sign -> LT x,0 */
if (arg_is_const_val(*p2, (ctx->type == TCG_TYPE_I32
? INT32_MIN : INT64_MIN))) {
/* TSTNE x,i -> LT x,0 if i only includes sign bit copies */
if (arg_is_const(*p2) && (arg_info(*p2)->val & ~i1->s_mask) == 0) {
*p2 = arg_new_constant(ctx, 0);
*pcond = tcg_tst_ltge_cond(cond);
return -1;