)]}'
{
  "commit": "e9b2e16dc98345bb1b91b1a6dacb3cec85f49e31",
  "tree": "40ff2a43efac29a276cb835af47b91d6d61608d3",
  "parents": [
    "a3fea0643da544b2e5d5c48f2ea942dad22d01f2"
  ],
  "author": {
    "name": "eddyz87",
    "email": "eddyz87@gmail.com",
    "time": "Mon Aug 19 19:49:11 2024 +0300"
  },
  "committer": {
    "name": "GitHub",
    "email": "noreply@github.com",
    "time": "Mon Aug 19 19:49:11 2024 +0300"
  },
  "message": "[BPF] introduce `__attribute__((bpf_fastcall))` (#101228)\n\nThis commit introduces attribute bpf_fastcall to declare BPF functions\r\nthat do not clobber some of the caller saved registers (R0-R5).\r\n\r\nThe idea is to generate the code complying with generic BPF ABI, but\r\nallow compatible Linux Kernel to remove unnecessary spills and fills of\r\nnon-scratched registers (given some compiler assistance).\r\n\r\nFor such functions do register allocation as-if caller saved registers\r\nare not clobbered, but later wrap the calls with spill and fill patterns\r\nthat are simple to recognize in kernel.\r\n\r\nFor example for the following C code:\r\n\r\n     #define __bpf_fastcall __attribute__((bpf_fastcall))\r\n\r\n     void bar(void) __bpf_fastcall;\r\n     void buz(long i, long j, long k);\r\n\r\n     void foo(long i, long j, long k) {\r\n       bar();\r\n       buz(i, j, k);\r\n     }\r\n\r\nFirst allocate registers as if:\r\n\r\n    foo:\r\n      call bar    # note: no spills for i,j,k (r1,r2,r3)\r\n      call buz\r\n      exit\r\n\r\nAnd later insert spills fills on the peephole phase:\r\n\r\n    foo:\r\n      *(u64 *)(r10 - 8) \u003d r1;  # Such call pattern is\r\n      *(u64 *)(r10 - 16) \u003d r2; # correct when used with\r\n      *(u64 *)(r10 - 24) \u003d r3; # old kernels.\r\n      call bar\r\n      r3 \u003d *(u64 *)(r10 - 24); # But also allows new\r\n      r2 \u003d *(u64 *)(r10 - 16); # kernels to recognize the\r\n      r1 \u003d *(u64 *)(r10 - 8);  # pattern and remove spills/fills.\r\n      call buz\r\n      exit\r\n\r\nThe offsets for generated spills/fills are picked as minimal stack\r\noffsets for the function. Allocated stack slots are not used for any\r\nother purposes, in order to simplify in-kernel analysis.\r\n\r\nCorresponding functionality had been merged in Linux Kernel as\r\n[this](https://lore.kernel.org/bpf/172179364482.1919.9590705031832457529.git-patchwork-notify@kernel.org/)\r\npatch set (the patch assumed that `no_caller_saved_regsiters` attribute\r\nwould be used by LLVM, naming does not matter for the Kernel).",
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