[v2] tcg/optimize: optimize TSTNE using smask and zmask

Commit Message

Paolo Bonzini Jan. 24, 2025, 10:27 a.m. UTC

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 <[e-mail]>
---
 tcg/optimize.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

Patch

diff --git a/tcg/optimize.c b/tcg/optimize.c
index c23f0d13929..a509acf53fe 100644
--- a/tcg/optimize.c
+++ b/tcg/optimize.c
@@ -112,6 +112,13 @@  static inline bool arg_is_const_val(TCGArg arg, uint64_t val)
     return ts_is_const_val(arg_temp(arg), val);
 }
 
+/* Calculate all the copies of the sign bit, both redundant and not. */
+static inline uint64_t all_sign_bit_copies(TCGType type, TempOptInfo *info)
+{
+    int64_t sign_bit = type == TCG_TYPE_I32 ? (int64_t)INT32_MIN : INT64_MIN;
+    return (info->s_mask >> 1) | sign_bit;
+}
+
 static inline bool ts_is_copy(TCGTemp *ts)
 {
     return ts_info(ts)->next_copy != ts;
@@ -765,6 +772,7 @@  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;
 
@@ -782,19 +790,22 @@  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 & ~all_sign_bit_copies(ctx->type, i1)) == 0) {
         *p2 = arg_new_constant(ctx, 0);
         *pcond = tcg_tst_ltge_cond(cond);
         return -1;