[Qemu-devel] [PATCH v2 09/11] target-arm: Eliminate unnecessary zero-extend in disas_bitfield

[Richard Henderson <rth@twiddle.net>]

For !SF, this initial ext32u can't be optimized away by the
current TCG code generator.  (It would require backward bit
liveness propagation.)

But since the range of bits for !SF are already constrained by
unallocated_encoding, we'll never reference the high bits anyway.

Signed-off-by: Richard Henderson <rth@twiddle.net>
---
 target-arm/translate-a64.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/target-arm/translate-a64.c b/target-arm/translate-a64.c
index 8c94edf..10f8825 100644
--- a/target-arm/translate-a64.c
+++ b/target-arm/translate-a64.c
@@ -2997,7 +2997,11 @@ static void disas_bitfield(DisasContext *s, uint32_t insn)
     }
 
     tcg_rd = cpu_reg(s, rd);
-    tcg_tmp = read_cpu_reg(s, rn, sf);
+
+    /* Suppress the zero-extend for !sf.  Since RI and SI are constrained
+       to be smaller than bitsize, we'll never reference data outside the
+       low 32-bits anyway.  */
+    tcg_tmp = read_cpu_reg(s, rn, 1);
 
     /* Recognize the common aliases.  */
     if (opc == 0) { /* SBFM */
-- 
2.4.3