Bazel build includes unused LLVM targets

[ScottTodd]

The upstream LLVM Bazel build has a list of LLVM targets that get unconditionally included in our builds: https://github.com/llvm/llvm-project/blob/009f88fc0e3a036be97ef7b222b90af342bae0b7/utils/bazel/llvm-project-overlay/llvm/BUILD.bazel#L1857-L2189 . These dramatically inflate build times, as compiling these targets is one of the slowest parts of an LLVM source build.

• AArch64
• ARM
• AMDGPU
• AVR
• BPF
• Hexagon
• Lanai
• LoongArch
• Mips
• MSP430
• NVPTX
• PowerPC
• Sparc
• SystemZ
• RISCV
• VE
• WebAssembly
• X86
• XCore

I'm seeing those targets get built with bazel build //tools:iree-opt. Could run some queries to see how the targets are specifically getting pulled in.

In our CMake build we conditionally enable targets based on configuration settings: https://github.com/openxla/iree/blob/d6357b4bbc55c9be36aa3bbe4c55f0fe1c2a010f/build_tools/cmake/iree_llvm.cmake#L128-L130
Bazel could either do the same with configs/selects, or it could at least be trimmed to just the set of targets that we use/support: https://github.com/openxla/iree/blob/d6357b4bbc55c9be36aa3bbe4c55f0fe1c2a010f/compiler/plugins/target/LLVMCPU/LLVMTargetOptions.cpp#L486-L494

[benvanik]

lol sparc and systemz

[ScottTodd]

Found where we can configure this: https://github.com/llvm/llvm-project/blob/main/utils/bazel/configure.bzl

DEFAULT_TARGETS = [
    "AArch64",
    "AMDGPU",
    "ARM",
    "AVR",
    "BPF",
    "Hexagon",
    "Lanai",
    "LoongArch",
    "Mips",
    "MSP430",
    "NVPTX",
    "PowerPC",
    "RISCV",
    "Sparc",
    "SystemZ",
    "VE",
    "WebAssembly",
    "X86",
    "XCore",
]
...

llvm_configure = repository_rule(
    implementation = _llvm_configure_impl,
    local = True,
    configure = True,
    attrs = {
        "targets": attr.string_list(default = DEFAULT_TARGETS),
    },
)

Called here:

iree/WORKSPACE

Line 42 in 7d823d2

llvm_configure(name = "llvm-project")