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Remove branch from combined_mode.

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On x86 it was probably only a conditional move, but this is fine.
This commit is contained in:
Thomas Harte 2022-04-22 15:11:41 -04:00
parent 4c806d7c51
commit 72772c9a83
1 changed files with 18 additions and 13 deletions
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31
InstructionSets/68k/Decoder.cpp
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@ -14,22 +14,27 @@ using namespace InstructionSet::M68k;
namespace {
constexpr AddressingMode extended_modes[] = {
AddressingMode::AbsoluteShort, // 1'000
AddressingMode::AbsoluteLong, // 1'001
AddressingMode::ProgramCounterIndirectWithDisplacement, // 1'010
AddressingMode::ProgramCounterIndirectWithIndex8bitDisplacement, // 1'011
AddressingMode::ImmediateData, // 1'100
AddressingMode::None, // [1'101]
AddressingMode::None, // [1'110]
AddressingMode::None, // [1'111]
};
/// @returns The @c AddressingMode given the specified mode and reg, subject to potential
/// aliasing on the '020+ as described above the @c AddressingMode enum.
constexpr AddressingMode combined_mode(int raw_mode, int reg) {
const auto mode = AddressingMode(raw_mode);
constexpr AddressingMode extended_modes[] = {
AddressingMode::AbsoluteShort,
AddressingMode::AbsoluteLong,
AddressingMode::ProgramCounterIndirectWithDisplacement,
AddressingMode::ProgramCounterIndirectWithIndex8bitDisplacement,
AddressingMode::ImmediateData,
constexpr AddressingMode combined_mode(int mode, int reg) {
assert(mode >= 0 && mode < 8);
assert(reg >= 0 && reg < 8);
AddressingMode::None,
AddressingMode::None,
AddressingMode::None,
};
return (raw_mode != 7) ? mode : extended_modes[reg];
// Look up mode, mapping invalid combinations to ::None, branchlessly.
const int use_reg = ((mode + 1) >> 3) & 1;
const AddressingMode modes[] = { AddressingMode(mode), extended_modes[reg] };
return modes[use_reg];
}
}