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chore: replace unnecessarily complex sign extension implementation

This commit is contained in:
Rekai Nyangadzayi Musuka 2022-02-28 20:38:50 -04:00
parent ddb68a7952
commit cb4d3a9a51
6 changed files with 19 additions and 41 deletions
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1
src/bus/dma.zig
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@ -85,7 +85,6 @@ pub fn DmaController(comptime id: u2) type {
const dad_adj = std.meta.intToEnum(Adjustment, self.cnt.dad_adj.read()) catch unreachable;
var offset: u32 = 0;
if (self.cnt.transfer_type.read()) {
offset = @sizeOf(u32); // 32-bit Transfer
const word = bus.read32(self._sad);

4
src/cpu/arm/branch.zig
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@ -4,13 +4,13 @@ const Bus = @import("../../Bus.zig");
const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
const InstrFn = @import("../../cpu.zig").ArmInstrFn;
const u32SignExtend = @import("../../util.zig").u32SignExtend;
const sext = @import("../../util.zig").sext;
pub fn branch(comptime L: bool) InstrFn {
return struct {
fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u32) void {
if (L) cpu.r[14] = cpu.r[15];
cpu.r[15] = cpu.fakePC() +% u32SignExtend(24, opcode << 2);
cpu.r[15] = cpu.fakePC() +% sext(24, opcode << 2);
}
}.inner;
}

8
src/cpu/arm/half_signed_data_transfer.zig
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@ -4,7 +4,7 @@ const Bus = @import("../../Bus.zig");
const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
const InstrFn = @import("../../cpu.zig").ArmInstrFn;
const u32SignExtend = @import("../../util.zig").u32SignExtend;
const sext = @import("../../util.zig").sext;
pub fn halfAndSignedDataTransfer(comptime P: bool, comptime U: bool, comptime I: bool, comptime W: bool, comptime L: bool) InstrFn {
return struct {
@ -42,14 +42,14 @@ pub fn halfAndSignedDataTransfer(comptime P: bool, comptime U: bool, comptime I:
},
0b10 => {
// LDRSB
result = u32SignExtend(8, bus.read8(address));
result = sext(8, bus.read8(address));
},
0b11 => {
// LDRSH
const value = if (address & 1 == 1) blk: {
break :blk u32SignExtend(8, bus.read8(address));
break :blk sext(8, bus.read8(address));
} else blk: {
break :blk u32SignExtend(16, bus.read16(address));
break :blk sext(16, bus.read16(address));
};
result = std.math.rotr(u32, value, 8 * (address & 1));

8
src/cpu/thumb/branch.zig
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@ -3,13 +3,13 @@ const Arm7tdmi = @import("../../cpu.zig").Arm7tdmi;
const InstrFn = @import("../../cpu.zig").ThumbInstrFn;
const checkCond = @import("../../cpu.zig").checkCond;
const u32SignExtend = @import("../../util.zig").u32SignExtend;
const sext = @import("../../util.zig").sext;
pub fn format16(comptime cond: u4) InstrFn {
return struct {
fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u16) void {
// B
const offset = u32SignExtend(8, opcode & 0xFF) << 1;
const offset = sext(8, opcode & 0xFF) << 1;
const should_execute = switch (cond) {
0xE, 0xF => cpu.panic("[CPU/THUMB.16] Undefined conditional branch with condition {}", .{cond}),
@ -27,7 +27,7 @@ pub fn format18() InstrFn {
return struct {
// B but conditional
fn inner(cpu: *Arm7tdmi, _: *Bus, opcode: u16) void {
const offset = u32SignExtend(11, opcode & 0x7FF) << 1;
const offset = sext(11, opcode & 0x7FF) << 1;
cpu.r[15] = (cpu.r[15] + 2) +% offset;
}
}.inner;
@ -47,7 +47,7 @@ pub fn format19(comptime is_low: bool) InstrFn {
cpu.r[14] = old_pc | 1;
} else {
// Instruction 1
cpu.r[14] = (cpu.r[15] + 2) +% (u32SignExtend(11, offset) << 12);
cpu.r[14] = (cpu.r[15] + 2) +% (sext(11, offset) << 12);
}
}
}.inner;

8
src/cpu/thumb/data_transfer.zig
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@ -14,7 +14,7 @@ pub fn format6(comptime rd: u3) InstrFn {
}.inner;
}
const u32SignExtend = @import("../../util.zig").u32SignExtend;
const sext = @import("../../util.zig").sext;
pub fn format78(comptime op: u2, comptime T: bool) InstrFn {
return struct {
@ -34,7 +34,7 @@ pub fn format78(comptime op: u2, comptime T: bool) InstrFn {
},
0b01 => {
// LDSB
cpu.r[rd] = u32SignExtend(8, bus.read8(address));
cpu.r[rd] = sext(8, bus.read8(address));
},
0b10 => {
// LDRH
@ -44,9 +44,9 @@ pub fn format78(comptime op: u2, comptime T: bool) InstrFn {
0b11 => {
// LDRSH
const value = if (address & 1 == 1) blk: {
break :blk u32SignExtend(8, bus.read8(address));
break :blk sext(8, bus.read8(address));
} else blk: {
break :blk u32SignExtend(16, bus.read16(address));
break :blk sext(16, bus.read16(address));
};
cpu.r[rd] = std.math.rotr(u32, value, 8 * (address & 1));

31
src/util.zig
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@ -1,29 +1,8 @@
const std = @import("std");
pub fn u32SignExtend(comptime bits: usize, value: u32) u32 {
return @bitCast(u32, signExtend(i32, bits, @bitCast(i32, value)));
}
fn signExtend(comptime T: type, comptime bits: usize, value: anytype) T {
const ValT = comptime @TypeOf(value);
comptime std.debug.assert(isInteger(ValT));
comptime std.debug.assert(isSigned(ValT));
const value_bits = @typeInfo(ValT).Int.bits;
comptime std.debug.assert(value_bits >= bits);
const bit_diff = value_bits - bits;
// (1 << bits) -1 is a mask that will take values like 0x100 and make them 0xFF
// value & mask so that only the relevant bits are sign extended
// therefore, value & ((1 << bits) - 1) is the isolation of the relevant bits
return ((value & ((1 << bits) - 1)) << bit_diff) >> bit_diff;
}
fn isInteger(comptime T: type) bool {
return @typeInfo(T) == .Int;
}
fn isSigned(comptime T: type) bool {
return @typeInfo(T).Int.signedness == .signed;
pub fn sext(comptime bits: comptime_int, value: u32) u32 {
comptime std.debug.assert(bits <= 32);
const amount = 32 - bits;
return @bitCast(u32, @bitCast(i32, value << amount) >> amount);
}