Remove special handling of bs_skip_bits2 for literal sizes

Starting from Erlang/OTP 26, the `bs_skip_bits2` instruction will almost never have a literal size in optimized (skip instructions with literal sizes will usually be part of a `bs_match` instruction). Therefore, there is no longer any point in handling literal sizes specially. Note that there was special handling of operands of `bs_skip_bits2` last emitted by Erlang/OTP 18. This is no longer necessary because the loader will refuse to load any module compiled by Erlang/OTP 22 or earlier.
bjorng · Jun 28, 2024 · 6f872f0 · 6f872f0
1 parent 9b04956
commit 6f872f0
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Showing 7 changed files with 27 additions and 101 deletions.
diff --git a/erts/emulator/beam/emu/bs_instrs.tab b/erts/emulator/beam/emu/bs_instrs.tab
@@ -280,17 +280,6 @@ i_bs_skip_bits2.execute(Fail, Unit) {
     }
 }
 
-i_bs_skip_bits_imm2(Fail, Ms, Bits) {
-    ErlSubBits *sb = (ErlSubBits*)bitstring_val($Ms);
-    size_t new_offset;
-    new_offset = sb->start + ($Bits);
-    if (new_offset <= sb->end) {
-        sb->start = new_offset;
-    } else {
-        $FAIL($Fail);
-    }
-}
-
 bs_init_writable() {
     HEAVY_SWAPOUT;
     x(0) = erts_bs_init_writable(c_p, x(0));

diff --git a/erts/emulator/beam/emu/ops.tab b/erts/emulator/beam/emu/ops.tab
@@ -1189,10 +1189,16 @@ i_bs_get_float2 xy f? t s t d
 
 # Miscellaneous
 
-bs_skip_bits2 Fail=f Ms=xy Sz=sq Unit=u Flags=u =>
-    skip_bits2(Fail, Ms, Sz, Unit, Flags)
+bs_skip_bits2 Fail=f Ms=xy Sz=xy Unit=u Flags=u =>
+    i_bs_skip_bits2 Ms Sz Fail Unit
+
+# Starting with Erlang/OTP 26, bs_skip_bits2 with an immediate size is
+# almost never found in optimized code. Therefore, there is no need to
+# to implement it efficiently.
+
+bs_skip_bits2 Fail=f Ms=xy Sz Unit=u Flags=u =>
+    move Sz x | bs_skip_bits2 Fail Ms x Unit Flags
 
-i_bs_skip_bits_imm2 f? xy W
 i_bs_skip_bits2 xy xy f? t
 
 bs_test_tail2 Fail=f Ms=xy o => jump Fail

diff --git a/erts/emulator/beam/generators.tab b/erts/emulator/beam/generators.tab
@@ -81,72 +81,6 @@ gen.get_utf16(Fail, Ms, Flags, Dst) {
     return op;
 }
 
-// Generate the fastest instruction for bs_skip_bits.
-gen.skip_bits2(Fail, Ms, Size, Unit, Flags) {
-    BeamOp* op;
-    $NewBeamOp(S, op);
-
-    $NativeEndian(Flags);
-    if (Size.type == TAG_a && Size.val == am_all) {
-        /*
-         * 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.
-         */
-        $BeamOpNameArity(op, bs_test_unit, 3);
-        op->a[0] = Fail;
-        op->a[1] = Ms;
-        op->a[2] = Unit;
-    } else if (Size.type == TAG_i) {
-        $BeamOpNameArity(op, i_bs_skip_bits_imm2, 3);
-        op->a[0] = Fail;
-        op->a[1] = Ms;
-        op->a[2].type = TAG_u;
-        if (!beam_load_safe_mul(Size.val, Unit.val, &op->a[2].val)) {
-            goto error;
-        }
-    } else if (Size.type == TAG_q) {
-        Eterm big = beamfile_get_literal(&S->beam, Size.val);
-        Uint bigval;
-
-        if (!term_to_Uint(big, &bigval)) {
-        error:
-            $BeamOpNameArity(op, jump, 1);
-            op->a[0] = Fail;
-        } else {
-            $BeamOpNameArity(op, i_bs_skip_bits_imm2, 3);
-            op->a[0] = Fail;
-            op->a[1] = Ms;
-            op->a[2].type = TAG_u;
-            if (!beam_load_safe_mul(bigval, Unit.val, &op->a[2].val)) {
-                goto error;
-            }
-        }
-    } else if (Size.type == TAG_x || Size.type == TAG_y) {
-        $BeamOpNameArity(op, i_bs_skip_bits2, 4);
-        op->a[0] = Ms;
-        op->a[1] = Size;
-        op->a[2] = Fail;
-        op->a[3] = Unit;
-    } else {
-        /*
-         * Invalid literal size. Can only happen if compiler
-         * optimizations are selectively disabled.  For example,
-         * at the time of writing, [no_copt, no_type_opt] will allow
-         * skip instructions with invalid sizes to slip through.
-         */
-        goto error;
-    }
-    return op;
-}
-
 // Creates an instruction that moves a literal lambda to a register.
 MakeLiteralFun(Op, Index, Arity, Dst) {
     SWord literal;

diff --git a/erts/emulator/beam/jit/arm/instr_bs.cpp b/erts/emulator/beam/jit/arm/instr_bs.cpp
@@ -510,13 +510,6 @@ void BeamModuleAssembler::emit_i_bs_skip_bits2(const ArgRegister &Ctx,
     }
 }
 
-void BeamModuleAssembler::emit_i_bs_skip_bits_imm2(const ArgLabel &Fail,
-                                                   const ArgRegister &Ctx,
-                                                   const ArgWord &Bits) {
-    mov_arg(ARG1, Bits);
-    emit_bs_skip_bits(Fail, Ctx);
-}
-
 void BeamModuleAssembler::emit_i_bs_get_binary2(const ArgRegister &Ctx,
                                                 const ArgLabel &Fail,
                                                 const ArgWord &Live,

diff --git a/erts/emulator/beam/jit/arm/ops.tab b/erts/emulator/beam/jit/arm/ops.tab
@@ -989,10 +989,16 @@ i_bs_get_float2 S f t s t d
 
 # Miscellaneous
 
-bs_skip_bits2 Fail=f Ms=xy Sz=sq Unit=u Flags=u =>
-    skip_bits2(Fail, Ms, Sz, Unit, Flags)
+bs_skip_bits2 Fail=f Ms=xy Sz=xy Unit=u Flags=u =>
+    i_bs_skip_bits2 Ms Sz Fail Unit
+
+# Starting with Erlang/OTP 26, bs_skip_bits2 with an immediate size is
+# almost never found in optimized code. Therefore, there is no need to
+# to implement it efficiently.
+
+bs_skip_bits2 Fail=f Ms=xy Sz Unit=u Flags=u =>
+    move Sz x | bs_skip_bits2 Fail Ms x Unit Flags
 
-i_bs_skip_bits_imm2 f S W
 i_bs_skip_bits2 S S f t
 
 bs_test_tail2 Fail=f Ms=xy o => jump Fail

diff --git a/erts/emulator/beam/jit/x86/instr_bs.cpp b/erts/emulator/beam/jit/x86/instr_bs.cpp
@@ -481,14 +481,6 @@ void BeamModuleAssembler::emit_i_bs_skip_bits2(const ArgRegister &Ctx,
     }
 }
 
-void BeamModuleAssembler::emit_i_bs_skip_bits_imm2(const ArgLabel &Fail,
-                                                   const ArgRegister &Ctx,
-                                                   const ArgWord &Bits) {
-    mov_arg(RET, Bits);
-
-    emit_bs_skip_bits(Fail, Ctx);
-}
-
 void BeamModuleAssembler::emit_i_bs_get_binary2(const ArgRegister &Ctx,
                                                 const ArgLabel &Fail,
                                                 const ArgWord &Live,

diff --git a/erts/emulator/beam/jit/x86/ops.tab b/erts/emulator/beam/jit/x86/ops.tab
@@ -911,10 +911,16 @@ i_bs_get_float2 S f t s t d
 
 # Miscellaneous
 
-bs_skip_bits2 Fail=f Ms=xy Sz=sq Unit=u Flags=u =>
-    skip_bits2(Fail, Ms, Sz, Unit, Flags)
+bs_skip_bits2 Fail=f Ms=xy Sz=xy Unit=u Flags=u =>
+    i_bs_skip_bits2 Ms Sz Fail Unit
+
+# Starting with Erlang/OTP 26, bs_skip_bits2 with an immediate size is
+# almost never found in optimized code. Therefore, there is no need to
+# to implement it efficiently.
+
+bs_skip_bits2 Fail=f Ms=xy Sz Unit=u Flags=u =>
+    move Sz x | bs_skip_bits2 Fail Ms x Unit Flags
 
-i_bs_skip_bits_imm2 f S W
 i_bs_skip_bits2 S S f t
 
 bs_test_tail2 Fail=f Ms=xy o => jump Fail