[Qemu-devel] [PATCH for-2.4] tcg/i386: Implement trunc_shr_i32

[Richard Henderson]

Enforce the invariant that 32-bit quantities are zero extended
in the register.  This avoids having to re-zero-extend at memory
accesses for 32-bit guests.

Signed-off-by: Richard Henderson <address@hidden>
---
Here's an alternative to the other things we've been considering.
We could even make this conditional on USER_ONLY if you like.

This does in fact fix the mips test case.  Consider the fact that
memory operations are probably more common than truncations, and
it would seem that we have a net size win by forcing the truncate
over adding a byte for the ADDR32 (or 2 bytes for a zero-extend).

Indeed, for 2.5, we could look at dropping the existing zero-extend
from the softmmu path.  Also for 2.5, split trunc_shr into two parts,
trunc_lo and trunc_hi.  No one really uses any other value for the
shift, and x86 could really use different constraints for the two.

Thoughts?

r~
---
 tcg/i386/tcg-target.c | 9 +++++++++
 tcg/i386/tcg-target.h | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
index ff4d9cf..9f01369 100644
--- a/tcg/i386/tcg-target.c
+++ b/tcg/i386/tcg-target.c
@@ -2040,6 +2040,14 @@ static inline void tcg_out_op(TCGContext *s, TCGOpcode 
opc,
     case INDEX_op_ext32s_i64:
         tcg_out_ext32s(s, args[0], args[1]);
         break;
+    case INDEX_op_trunc_shr_i32:
+        if (args[2] > 0) {
+            tcg_out_shifti(s, SHIFT_SHR + P_REXW, args[0], args[2]);
+        }
+        if (args[2] < 32) {
+            tcg_out_ext32u(s, args[0], args[0]);
+        }
+        break;
 #endif
 
     OP_32_64(deposit):
@@ -2170,6 +2178,7 @@ static const TCGTargetOpDef x86_op_defs[] = {
     { INDEX_op_ext8u_i64, { "r", "r" } },
     { INDEX_op_ext16u_i64, { "r", "r" } },
     { INDEX_op_ext32u_i64, { "r", "r" } },
+    { INDEX_op_trunc_shr_i32,  { "r", "0" } },
 
     { INDEX_op_deposit_i64, { "Q", "0", "Q" } },
     { INDEX_op_movcond_i64, { "r", "r", "re", "r", "0" } },
diff --git a/tcg/i386/tcg-target.h b/tcg/i386/tcg-target.h
index 25b5133..8b87050 100644
--- a/tcg/i386/tcg-target.h
+++ b/tcg/i386/tcg-target.h
@@ -102,7 +102,7 @@ extern bool have_bmi1;
 #define TCG_TARGET_HAS_mulsh_i32        0
 
 #if TCG_TARGET_REG_BITS == 64
-#define TCG_TARGET_HAS_trunc_shr_i32    0
+#define TCG_TARGET_HAS_trunc_shr_i32    1
 #define TCG_TARGET_HAS_div2_i64         1
 #define TCG_TARGET_HAS_rot_i64          1
 #define TCG_TARGET_HAS_ext8s_i64        1
-- 
2.4.3