Refine the build system

[DrXiao]

This pull request resolves the following issues:

Fix empty TARGET_EXEC variable

Previously, I noticed that the build system directly executed the stage1 compiler to build the stage2 compiler, instead of invoking qemu-user. This issue could be observed by running make VERBOSE=1

$ make VERBOSE=1
...
...
+out/shecc-stage1.elf -o out/shecc-stage2.elf src/main.c
...

That is, TARGET_EXEC, which stores the the name of an architecture-specific qemu executable, is left empty and not properly assigned.

Therefore, these changes fix it to ensure the build system invokes qemu as expected.

Close #44

Summary by Bito

This pull request refines the build system by fixing the empty TARGET_EXEC variable issue, enhancing Makefiles for ARM and RISC-V with prerequisite checks, and improving commands for invoking qemu executables for better cross-compilation reliability.

Unit tests added: False

Estimated effort to review (1-5, lower is better): 2

[jserv]

If target architecture does not match host (e.g., configuring Arm32 on x86-64), the build system should warn and stop at stage 0.

[DrXiao]

If target architecture does not match host (e.g., configuring Arm32 on x86-64), the build system should warn and stop at stage 0.

I am confused by this description and not sure if I understood correctly. If we configure Arm32 on x86-64, the build system should proceed with bootstrapping. The stage 2 compiler would be built by the stage 1 compiler running through qemu-arm, rather than stopping the build.

[jserv]

If we configure Arm32 on x86-64, the build system should proceed with bootstrapping.

Yes, it is valid when qemu-arm or qemu-riscv32 emulators are properly configured in the environment. However, the current implementation lacks comprehensive documentation or messages that would explain the emulation process, expected outputs, and potential limitations. Adding verbose explanations would significantly improve usability and troubleshooting capabilities for developers working across different architectures.

[DrXiao]

For the current build system, consider the following table, which lists the valid host-target combinations for building shecc:

host architecture	target architecture for shecc	Remark
x86-64	armv7	use qemu-arm to emulate
x86-64	riscv	use qemu-riscv to emulate
armv7	armv7	run directly on the host computer

Adding verbose explanations would significantly improve usability and troubleshooting capabilities for developers working across different architectures.

If I understood correctly, for example:

• Use a computer with an armv7 architecture to build shecc targeting riscv.
• Use a computer with a powerpc architecture to build shecc targeting armv7 or riscv.

The above builds should be considered invalid because these host-target combinations are not listed in the table.

That is, in the first case, we do not allow developers to build shecc targeting riscv on an ARM machine. In the second case, we also do not allow developers build shecc targeting arm or riscv on a non-x86-64 machine.

Therefore, in such cases, the build system should stop and output a warning message to inform the developer.

Is my understanding correct?

[jserv]

Is my understanding correct?

Yes, all architectures/platforms are capable of running stage0 once an ordinary C compiler is found. There is explicitly a notable prerequisite to enter stage1 and eventually the following stage2.

[DrXiao]

I have adjusted the validation approach. The build system will check the required packages and print the warning messages when make config. However, if qemu is required but missing, the build will proceed up to stage 0, print warning messages, and stop at the beginning of stage 1.

I tested the behavior using my beaglebone black (running Debian OS) by running make ARCH=riscv, and the build system behaved as expected:

debian@BeagleBone:~/shecc$ make ARCH=riscv
env printf "ARCH=riscv" > .session.mk
Target machine code switch to riscv
Warning: missing packages: dot jq qemu-riscv32
Warning: Please check package installation
  CC+LD out/inliner
  GEN   out/libc.inc
  CC    out/src/main.o
  LD    out/shecc
Warning: failed to build the stage 1 and stage 2 compilers due to missing qemu-riscv32
make: *** [Makefile:108: out/shecc-stage1.elf] Error 1

If running make again, the build system simply shows the warning.

debian@BeagleBone:~/shecc$ make
Warning: failed to build the stage 1 and stage 2 compilers due to missing qemu-riscv32
make: *** [Makefile:108: out/shecc-stage1.elf] Error 1

In addition, becasue the Debian system does not have graphviz and jq, the relevant warning messages (Warning: missing packages: dot jq qemu-riscv32) are also printed during make config.

[jserv]

Both mk/arm.mk and mk/riscv.mk have common rules, and it is better to use define to unify them.

[jserv]

Thank @DrXiao for contributing!