Add fcontext support for DEC Alpha (SysV/ELF) - #342
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Alpha had no fcontext assembly, so a build on that architecture found no matching asm_sources alternative. The Alpha calling standard returns a 16 byte struct such as transfer_t through a hidden pointer passed in $16, which shifts the declared arguments to $17 and $18. That pointer belongs to the frame that made the call, so it has to travel with the context: each context records the pointer its own suspending call was given, and whoever resumes that context stores the transfer_t through it. Among the existing ports only ppc32-sysv-linux returns a context through memory this way. transfer_t is also passed by value in a register pair, so $16 and $17 are loaded with it as well. A context entered for the first time by make_fcontext reads its argument that way; for an ordinary resumption those are argument registers the resumed call may clobber, so setting them unconditionally is harmless. The resumption PC is loaded into $27 so that it doubles as the procedure value a context-function entered for the first time computes its own gp from. The transfer at the end of jump_fcontext and ontop_fcontext uses the ret hint rather than jmp. Those functions are entered by a bsr, which pushes onto the return prediction stack, and the resumption PC is where that call appears to return, so a jmp hint would orphan an entry per switch; the ret hint pops it. Worth about 3% on a ping-pong switch benchmark on an EV68AL UP1500. jsr_coroutine does not fit despite the name: it pushes PC+4, and resume points live in the context-data.
The CMake build selects the fcontext assembly by interpolating BOOST_CONTEXT_ARCHITECTURE into the source name, and alpha was not among the recognized values, so a CMake build on Alpha fell through to the 64-bit default of x86_64. Listing alpha in _all_archs is enough: CMAKE_SYSTEM_PROCESSOR is "alpha" there, so the IN_LIST branch matches it, as it already does for riscv64, s390x and loongarch64. The remaining defaults are sysv, elf and gas.
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Adds the three fcontext assembly routines for Alpha on Linux (SysV ABI, ELF), plus the b2 and CMake wiring and a doc table entry.
src/asm/{make,jump,ontop}_alpha_sysv_elf_gas.Sbuild/Jamfile.v2:asm_sourcesalternative for<architecture>alpha <address-model>64 <abi>sysv <binary-format>elfCMakeLists.txt:alphaadded to_all_archs;CMAKE_SYSTEM_PROCESSORisalpha, so the existingIN_LISTbranch matches it the same way riscv64/s390x/loongarch64 are already handleddoc/architectures.qbk: table rowABI note
The Alpha calling standard returns a 16-byte struct such as
transfer_tthrough a hidden pointer in$16, which shifts the declared arguments to$17/$18. That pointer belongs to the frame that made the call, so it has to travel with the context: each context records the pointer its own suspending call was given, and whoever resumes it stores thetransfer_tthrough it. Among the existing ports only ppc32-sysv-linux returns a context through memory this way, so this is the main thing worth reviewing.transfer_tis also passed by value in$16/$17, which is how a context entered for the first time bymake_fcontextreads its argument.Branch prediction
The transfers end in
ret $31, ($27), 0rather thanjmp. The Alpha jump-format opcodes are architecturally identical and differ only in two hint bits driving the return prediction stack.jump_fcontextis entered by absr, which pushes, and the resumption PC is where that call appears to return — so ajmphint orphans one entry per switch, whileretpops it. Measured 131.8 -> 128.1 ns/switch (-2.9%) on an EV68AL UP1500, winning all 16 interleaved runs across call depths 0/4/8/16.(
jsr_coroutinedoes not fit despite the name: its push value is architecturallyPC+4, and fcontext resume points live in the saved context-data, never atPC+4.)Testing
Assembled, linked and run on real hardware (UP1500, EV68AL, gcc 16.2.0) and under
qemu-alpha:jump_fcontext:$9-$15and$f2-$f9, using values kept live across the call so the compiler allocates them naturally rather than relying on asm clobbersDepends on
Architecture detection lands elsewhere;
<architecture>alphadoes not exist without these, so this PR is not usable on its own:alphato the<architecture>featurealphato the architecture check listchecks/architecture/alpha.cpp