From b96e5bd87c6aaaf96fa9c6e3679d95df74d1a499 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Gustavsson?= <bjorn@erlang.org>
Date: Tue, 5 Mar 2019 05:44:48 +0100
Subject: Eliminate unused i_bs_skip_bits_all2 instruction

Starting in OTP 19 (in commit 9504c0dd71d0), the compiler emits
a test_unit instruction instead of a skip instruction at the end
of binary. We can do the same replacement in the loader to get
rid of the i_bs_skip_bits_all2 instruction.
---
 erts/emulator/beam/beam_load.c   | 16 ++++++++++++++--
 erts/emulator/beam/bs_instrs.tab | 10 ----------
 erts/emulator/beam/ops.tab       |  1 -
 3 files changed, 14 insertions(+), 13 deletions(-)

(limited to 'erts/emulator/beam')

diff --git a/erts/emulator/beam/beam_load.c b/erts/emulator/beam/beam_load.c
index 5aaba78de4..e3232f1beb 100644
--- a/erts/emulator/beam/beam_load.c
+++ b/erts/emulator/beam/beam_load.c
@@ -3565,10 +3565,22 @@ gen_skip_bits2(LoaderState* stp, GenOpArg Fail, GenOpArg Ms,
     NATIVE_ENDIAN(Flags);
     NEW_GENOP(stp, op);
     if (Size.type == TAG_a && Size.val == am_all) {
-	op->op = genop_i_bs_skip_bits_all2_3;
+        /*
+         * This kind of skip instruction will only be found in modules
+         * compiled before OTP 19. From OTP 19, the compiler generates
+         * a test_unit instruction of a bs_skip at the end of a
+         * binary.
+         *
+         * It is safe to replace the skip instruction with a test_unit
+         * instruction, because the position will never be used again.
+         * If the match context itself is used again, it will be used by
+         * a bs_restore2 instruction which will overwrite the position
+         * by one of the stored positions.
+         */
+	op->op = genop_bs_test_unit_3;
 	op->arity = 3;
 	op->a[0] = Fail;
-	op->a[1] = Ms; 
+	op->a[1] = Ms;
 	op->a[2] = Unit;
     } else if (Size.type == TAG_i) {
 	op->op = genop_i_bs_skip_bits_imm2_3;
diff --git a/erts/emulator/beam/bs_instrs.tab b/erts/emulator/beam/bs_instrs.tab
index 4cf7faffb7..10f43cd786 100644
--- a/erts/emulator/beam/bs_instrs.tab
+++ b/erts/emulator/beam/bs_instrs.tab
@@ -272,16 +272,6 @@ i_bs_skip_bits2.execute(Fail, Unit) {
     }
 }
 
-i_bs_skip_bits_all2(Fail, Ms, Unit) {
-    ErlBinMatchBuffer *_mb;
-    _mb = ms_matchbuffer($Ms);
-    if (((_mb->size - _mb->offset) % $Unit) == 0) {
-        _mb->offset = _mb->size;
-    } else {
-        $FAIL($Fail);
-    }
-}
-
 i_bs_skip_bits_imm2(Fail, Ms, Bits) {
     ErlBinMatchBuffer *_mb;
     size_t new_offset;
diff --git a/erts/emulator/beam/ops.tab b/erts/emulator/beam/ops.tab
index 2f00b82c1e..4e7e5f5de1 100644
--- a/erts/emulator/beam/ops.tab
+++ b/erts/emulator/beam/ops.tab
@@ -1192,7 +1192,6 @@ bs_skip_bits2 Fail=f Ms=xy Sz=sq Unit=u Flags=u => \
 
 i_bs_skip_bits_imm2 f? xy W
 i_bs_skip_bits2 xy xy f? t
-i_bs_skip_bits_all2 f? xy t
 
 bs_test_tail2 Fail=f Ms=xy Bits=u==0 => bs_test_zero_tail2 Fail Ms
 bs_test_tail2 Fail=f Ms=xy Bits=u => bs_test_tail_imm2 Fail Ms Bits
-- 
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