When it comes to choosing an ARM compiler, developers typically have three main options – IAR, GNU, and LLVM/Clang. Each compiler has its own strengths and weaknesses, and the right choice depends on your specific needs and priorities. This comprehensive guide examines the key factors to consider when comparing these three ARM compilers.
Background
ARM processors power over 95% of smartphones and a large percentage of embedded devices. To program these devices, developers need an ARM compiler that converts human-readable code into machine code that the processor can execute. The three most common choices are:
- IAR – A proprietary compiler from IAR Systems known for performance and optimization.
- GNU – The free, open source compiler from the GNU Project, part of the GCC compiler collection.
- LLVM/Clang – An open source compiler infrastructure spearheaded by Apple focused on speed and modularity.
While technical factors like code optimization and compiler speed are critical, there are also business factors to consider such as licensing models, support options, and integration with preferred IDEs. We’ll explore all of these facets and more in this comprehensive comparison.
Code Quality and Optimization
For most developers, generating high performance object code is a top priority. Compiler optimizations like function inlining, dead code elimination, and efficient register allocation can create significant performance improvements. Here’s how the top ARM compilers compare when it comes to code quality:
IAR
IAR consistently benchmarks near or at the top for code efficiency among ARM compilers. Developers praise the high optimization levels and aggressive algorithms that produce faster and smaller code. IAR offers distinct optimization settings for speed or size depending on your priority. The proprietary optimizations and back-end designed solely for ARM give IAR an edge.
GNU
The GNU Compiler Collection includes leading open source compilers like GCC and G++. The code quality is generally very good, but GCC wasn’t originally designed just for ARM so optimizations may not be as aggressive as IAR’s. GNU offers the standard optimization levels O1 through O3. While GCC generates efficient code, IAR regularly benchmarks at 10-15% faster for certain benchmarks.
LLVM/Clang
LLVM and Clang aim to provide an optimizing compiler framework designed for compile speeds, better diagnostics, and modularity. Clang is the front-end that handles parsing and code generation, while LLVM contains the middle and back-ends with optimizations. Some tests show LLVM generating faster code than GCC for ARM. Overall LLVM is optimized more for compilation speed rather than the highest performing end code.
Verdict
IAR has a slight edge when it comes to sheer code efficiency, but the other compilers are quite capable as well. For extremely performance sensitive applications, IAR stands out. But GNU and LLVM/Clang offer very competitive code generation while prioritizing other areas like compilation speed or licensing.
Compilation Speed
For large projects or those with frequent code changes, slow compile times can quickly become problematic. Here are some key considerations for compilation speed with each compiler:
IAR
IAR puts a priority on code efficiency over raw compile speed. While optimizations certainly help runtime performance, they generally increase compile time. IAR’s strongest optimizations will compile slower than rivals. However, the highly optimized backend also allows some optimizations that actually improve speed.
GNU
GCC and G++ have put significant work into improving compile times while maintaining code quality. Concurrency and parallelism during compiles helps GNU achieve faster average speeds than many other compilers. The compilation process itself contains optimizations to reduce unnecessary work. GNU also allows flexible build configuration options to tradeoff between speed and size.
LLVM/Clang
LLVM and Clang set themselves apart with an architecture optimized for faster compiles. Features like incremental compilation avoid redundant passes over unchanged code. LLVM’s modularity also allows different optimization levels to better balance speed and code quality. For large projects, LLVM/Clang averages substantially faster compile speeds.
Verdict
GNU and LLVM/Clang have significant advantages in compilation speed over IAR in most cases. LLVM’s architecture gives it an edge in average times for large projects. However, IAR’s backend can offset some slowdowns during optimization. Compile time depends heavily on code complexity and optimization levels.
IDE Integration
A quality integrated development environment (IDE) improves programmer productivity and satisfaction. Tight integration with the compiler and debugging tools is highly valuable. Let’s look at IDE support for each compiler family:
IAR
IAR Embedded Workbench is the IDE developed by IAR specifically for their toolchain. As an integrated solution, IAR provides excellent support and integration out of the box. Preconfigured project templates and 100% compatibility with their compilers and debuggers makes for a streamlined workflow.
GNU
GCC and G++ work well with IDEs like Eclipse, Visual Studio Code, and CLion. However, some integration and configuration may be required. GNU relies more on open source project plugins and extensions rather than complete IDE products. This provides flexibility at the cost of out-of-box convenience.
LLVM/Clang
LLVM and Clang also integrate into popular IDEs like Xcode, Eclipse, and Visual Studio. Their focus on modularity improves ability to embed components in different environments. However, some integration work may be needed to tie LLVM and Clang into the complete toolchain.
Verdict
IAR Embedded Workbench provides the tightest integrated development environment with its compilers. GNU and LLVM offer flexibility but may require more manual configuration and plugins to achieve the same integration levels.
Debugging Capabilities
Robust debugging improves productivity by quickly locating bugs and reducing new code defects. Let’s explore the debugging features for each compiler option:
IAR
IAR Embedded Workbench includes both C-SPY simulator-based debugging as well as J-Link hardware debugging support. The technology integrates well for trace capability, profiling, and code coverage analysis. Seamless coordination between compiler and debugger speeds up finding and fixing issues.
GNU
The Eclipse IDE and GDB GNU debugger offer full-featured debugging for GNU’s toolchains. GDB connects to ARM’s CoreSight on-chip debug modules for hardware debugging and visibility. A wide range of GDB frontends and IDE integration allows debugging flexibility for developers.
LLVM/Clang
LLDB is LLVM’s debugger of choice which also integrates with IDEs like Xcode and Visual Studio Code. It supports all standard debugging features and connects to hardware for JTAG and SWD interfaces. Plugin extensions help tailor LLDB to specific environments and use cases.
Verdict
All three compiler families provide capable debugging either through software simulation or JTAG/SWD hardware. IAR’s solution feels slightly more integrated end-to-end, but the open source options offer excellent debugging too with more IDE options.
Technical Support and Training
Getting prompt answers to questions and issues is critical, especially for production development. Let’s contrast the support models for these compilers:
IAR
As a commercial toolchain, IAR offers multiple premium technical support and maintenance packages. These include email, phone, and live chat options with guaranteed response times. IAR also provides detailed documentation and self-help resources online.
GNU
Troubleshooting for GCC and other GNU tools relies heavily on community-based support through channels like Stack Overflow. Paid commercial support from third parties is available too. GNU’s open source documentation is extensive but can sometimes be outdated.
LLVM/Clang
LLVM uses mailing lists and web forums for project support along with an open bug tracker. Documentation is generally good although oriented more toward compiler development rather than end users. Paid support plans are not offered.
Verdict
For mission-critical commercial projects, IAR’s professional support services are worthwhile. The open source tools offer community-based help and documentation but lack any guarantee or service level agreements.
Licensing and Cost
Development costs include both the compiler toolchain licensing and hardware debugging hardware. Here are the cost considerations for each option:
IAR
IAR offers scalable annual licensing based on number of developers and compiler features needed. For example, a single-user license starts around $4000 per year. Budget-conscious developers can use a node-locked or time-limited license for under $2000. Student licenses are also available.
GNU
GCC, GDB, and associated GNU tools are licensed under GPL and other open source licenses. This allows free use and distribution. Paid commercial support, services, and warranties are available optionally through third parties.
LLVM/Clang
As an open source project, LLVM and Clang incur no licensing fees. The compiler and tools can be used without charge. Optional paid support is available from companies like Arm, Intel, and Apple that rely on LLVM.
Verdict
IAR provides a proven, reliable toolchain with premium support, at significant cost. The open source options forfeit support but are highly capable at no monetary charge. For startups and those highly budget-conscious, avoiding annual licensing fees may outweigh other considerations. For organizations that can afford the investment, IAR provides extra confidence, performance, and support.
ARM Processor Compatibility
Compiler support for the specific ARM architecture you’re using is essential. Let’s look at compatibility:
IAR
IAR supports the complete range of ARM Cortex devices as well as legacy ARM7, ARM9, and ARM11 processors. Their compilers are specifically optimized for ARM rather than being ported from other architectures. IAR also has early access to new ARM cores for quick toolchain updates.
GNU
GCC, G++, and GDB all support ARM architectures back to ARMv4 introduced in the late 90’s. Support extends up through the latest ARMv8 64-bit cores. The open source model sometimes results in bigger gaps after new ARM core releases.
LLVM/Clang
LLVM and Clang also support ARM CPUs starting from ARMv4 up through ARMv8 and the newest Cortex-A, -R, and -M processor variants. Apple’s extensive use of LLVM benefits ARM compatibility. As with GNU, support for bleeding-edge ARM cores takes longer.
Verdict
All three compilers support the common ARM architectures and cores, from legacy to the newest 64-bit designs. IAR sometimes has an advantage supporting the newest ARM releases sooner thanks to early access and a targeted toolchain.
Conclusion
IAR, GNU, and LLVM/Clang each have compelling advantages making them suitable for different development needs:
- IAR offers best-in-class optimizations paired with excellent support and integration. The premium toolchain is worth the cost for mission-critical projects.
- GNU provides no-cost access to capable compilers with good code generation and faster compile speeds.
- LLVM prioritizes compiler architecture and faster compilation while still producing high quality code.
There is no unambiguous “best” ARM compiler. Developers should weigh factors like performance, cost, support needs, and licensing models. For commercially-supported tools with aggressive optimizations, IAR excels. To move fast with no licensing costs, LLVM or GNU are great open source options. Combining the strengths of multiple compilers is also an option. There are advantages to each compiler that make it suitable for various use cases.
