[Tarantool-patches] [PATCH luajit 1/2] ARM64: Make tobit conversions match JIT backend behavior.

[Sergey Kaplun via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

From: Mike Pall <mike>

Thanks to Peter Cawley.

(cherry picked from commit cdc2db3aeac442e139c3b1fda715fdfa78da8791)

On the aarch64 architecture, the `tobit()` function incorrectly
truncates instead of rounding to the nearest integer. This leads to
inconsistency between various VMs (comparing to x86/x64, for example)
and between the arm64 VM and JIT.

This patch uses an addition of 2^52 + 2^51 constant to the given
argument to convert it to the nearest integer value. After reading the
lowest 32 bits of the register, we get the expected result.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#12880
---
 src/vm_arm64.dasc                             | 20 ++++---------
 .../lj-1253-tobit-conversion.test.lua         | 28 +++++++++++++++++++
 2 files changed, 34 insertions(+), 14 deletions(-)
 create mode 100644 test/tarantool-tests/lj-1253-tobit-conversion.test.lua

diff --git a/src/vm_arm64.dasc b/src/vm_arm64.dasc
index 57131140..4ffd7378 100644
--- a/src/vm_arm64.dasc
+++ b/src/vm_arm64.dasc
@@ -1613,22 +1613,14 @@ static void build_subroutines(BuildCtx *ctx)
   |
   |//-- Bit library --------------------------------------------------------
   |
-  |// FP number to bit conversion for soft-float. Clobbers CARG1-CARG3
+  |// FP number to bit conversion. Clobbers CARG1-CARG2, FARG1-FARG2.
   |->vm_tobit_fb:
   |  bls ->fff_fallback
-  |  add CARG2, CARG1, CARG1
-  |  mov CARG3, #1076
-  |  sub CARG3, CARG3, CARG2, lsr #53
-  |  cmp CARG3, #53
-  |  bhi >1
-  |  and CARG2, CARG2, #U64x(001fffff,ffffffff)
-  |  orr CARG2, CARG2, #U64x(00200000,00000000)
-  |   cmp CARG1, #0
-  |  lsr CARG2, CARG2, CARG3
-  |   cneg CARG1w, CARG2w, mi
-  |  br lr
-  |1:
-  |  mov CARG1w, #0
+  |  fmov FARG1, CARG1
+  |  movz CARG2, #0x4338, lsl #48
+  |  fmov FARG2, CARG2
+  |  fadd FARG1, FARG1, FARG2
+  |  fmov CARG1w, s0
   |  br lr
   |
   |.macro .ffunc_bit, name
diff --git a/test/tarantool-tests/lj-1253-tobit-conversion.test.lua b/test/tarantool-tests/lj-1253-tobit-conversion.test.lua
new file mode 100644
index 00000000..31cf97ca
--- /dev/null
+++ b/test/tarantool-tests/lj-1253-tobit-conversion.test.lua
@@ -0,0 +1,28 @@
+local tap = require('tap')
+
+-- Test file to demonstrate LuaJIT's incorrect bit.tobit
+-- behaviour for arm64.
+-- See also: https://github.com/LuaJIT/LuaJIT/issues/1253.
+
+local test = tap.test('lj-1253-tobit-conversion')
+
+test:plan(2)
+
+test:is(bit.tobit(1.7), 2, 'correct bit.tobit rounding')
+
+test:skipcond({
+  ['Test requires JIT enabled'] = not jit.status(),
+})
+
+local results = {}
+
+jit.opt.start('hotloop=1')
+
+for i = 1, 4 do
+  -- Use constants on trace.
+  results[i] = bit.tobit(1.7)
+end
+
+test:samevalues(results, 'consistent JIT and VM behaviour for bit.tobit')
+
+test:done(true)
-- 
2.55.0