target/arm: Implement v8.1M low-overhead-loop instructions

v8.1M's "low-overhead-loop" extension has three instructions
for looping:
 * DLS (start of a do-loop)
 * WLS (start of a while-loop)
 * LE (end of a loop)

The loop-start instructions are both simple operations to start a
loop whose iteration count (if any) is in LR.  The loop-end
instruction handles "decrement iteration count and jump back to loop
start"; it also caches the information about the branch back to the
start of the loop to improve performance of the branch on subsequent
iterations.

As with the branch-future instructions, the architecture permits an
implementation to discard the LO_BRANCH_INFO cache at any time, and
QEMU takes the IMPDEF option to never set it in the first place
(equivalent to discarding it immediately), because for us a "real"
implementation would be unnecessary complexity.

(This implementation only provides the simple looping constructs; the
vector extension MVE (Helium) adds some extra variants to handle
looping across vectors.  We'll add those later when we implement
MVE.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201019151301.2046-8-peter.maydell@linaro.org

1 files changed, 8 insertions, 0 deletions

diff --git a/target/arm/t32.decode b/target/arm/t32.decode
index 3015731a8d..8152739b52 100644
--- a/target/arm/t32.decode
+++ b/target/arm/t32.decode
@@ -659,4 +659,12 @@ BL               1111 0. .......... 11.1 ............         @branch24
     BF           1111 0 boff:4 10 ----- 1110 - ---------- 1    # BF
     BF           1111 0 boff:4 11 ----- 1110 0 0000000000 1    # BFX, BFLX
   ]
+  [
+    # LE and WLS immediate
+    %lob_imm 1:10 11:1 !function=times_2
+
+    DLS          1111 0 0000 100     rn:4 1110 0000 0000 0001
+    WLS          1111 0 0000 100     rn:4 1100 . .......... 1 imm=%lob_imm
+    LE           1111 0 0000 0 f:1 0 1111 1100 . .......... 1 imm=%lob_imm
+  ]
 }