On Thu, Feb 13, 2025 at 1:58 AM H.J. Lu <hjl.to...@gmail.com> wrote:
>
> x86 conditional branch (jcc) target can be either a label or a symbol.
> Add a pass to fold tail call with jcc by turning:
>
> jcc .L6
> ...
> .L6:
> jmp tailcall
>
> into:
>
> jcc tailcall
>
> After basic block reordering pass, conditional branches look like
>
> (jump_insn 7 6 14 2 (set (pc)
> (if_then_else (eq (reg:CCZ 17 flags)
> (const_int 0 [0]))
> (label_ref:DI 23)
> (pc))) "x.c":8:5 1458 {jcc}
> (expr_list:REG_DEAD (reg:CCZ 17 flags)
> (int_list:REG_BR_PROB 217325348 (nil)))
> ...
> (code_label 23 20 8 4 4 (nil) [1 uses])
> (note 8 23 9 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
> (call_insn/j 9 8 10 4 (call (mem:QI (symbol_ref:DI ("bar") [flags 0x41]
> <functi
> on_decl 0x7f4cff3c0b00 bar>) [0 bar S1 A8])
> (const_int 0 [0])) "x.c":8:14 discrim 1 1469 {sibcall_di}
> (expr_list:REG_CALL_DECL (symbol_ref:DI ("bar") [flags 0x41]
> <function_dec
> l 0x7f4cff3c0b00 bar>)
> (nil))
> (nil))
>
> If the branch edge destination is a basic block with only a direct
> sibcall, change the jcc target to the sibcall target and decrement
> the destination basic block entry label use count. Even though the
> destination basic block is unused, it must be kept since it is required
> by RTL control flow check and JUMP_LABEL of the conditional jump can
> only point to a code label, not a code symbol. Dummy sibcall patterns
> are added so that sibcalls in basic blocks, whose entry label use count
> is 0, won't be generated.
This reads like you are trying to get around some checks in RTL
control flow. So, either changes you are performing to RTX stream are
not allowed (these checks are here for a reason), or the
infrastructure is not (yet) prepared to handle this functionality.
Either way, please discuss with infrastructure maintainers (CC'd)
first if the approach is correct and if these changes to RTX stream
are allowed by the infra.
Thanks,
Uros.
>
> Jump tables like
>
> foo:
> .cfi_startproc
> cmpl $4, %edi
> ja .L1
> movl %edi, %edi
> jmp *.L4(,%rdi,8)
> .section .rodata
> .L4:
> .quad .L8
> .quad .L7
> .quad .L6
> .quad .L5
> .quad .L3
> .text
> .L5:
> jmp bar3
> .L3:
> jmp bar4
> .L8:
> jmp bar0
> .L7:
> jmp bar1
> .L6:
> jmp bar2
> .L1:
> ret
> .cfi_endproc
>
> can also be changed to:
>
> foo:
> .cfi_startproc
> cmpl $4, %edi
> ja .L1
> movl %edi, %edi
> jmp *.L4(,%rdi,8)
> .section .rodata
> .L4:
> .quad bar0
> .quad bar1
> .quad bar2
> .quad bar3
> .quad bar4
> .text
> .L1:
> ret
> .cfi_endproc
>
> After basic block reordering pass, jump tables look like:
>
> (jump_table_data 16 15 17 (addr_vec:DI [
> (label_ref:DI 18)
> (label_ref:DI 22)
> (label_ref:DI 26)
> (label_ref:DI 30)
> (label_ref:DI 34)
> ]))
> ...
> (code_label 30 17 31 4 5 (nil) [1 uses])
> (note 31 30 32 4 [bb 4] NOTE_INSN_BASIC_BLOCK)
> (call_insn/j 32 31 33 4 (call (mem:QI (symbol_ref:DI ("bar3") [flags 0x41]
> <function_decl 0x7f21be3c0e00 bar3>) [0 bar3 S1 A8])
> (const_int 0 [0])) "j.c":15:13 1469 {sibcall_di}
> (expr_list:REG_CALL_DECL (symbol_ref:DI ("bar3") [flags 0x41]
> <function_decl 0x7f21be3c0e00 bar3>)
> (nil))
> (nil))
>
> If the jump table entry points to a target basic block with only a direct
> sibcall, change the entry to point to the sibcall target and decrement
> the target basic block entry label use count. If the target basic block
> isn't kept for JUMP_LABEL of the conditional tailcall, delete it if its
> entry label use count is 0.
>
> Update final_scan_insn_1 to skip a label if its use count is 0 and
> support symbol reference in jump table. Update create_trace_edges to
> skip symbol reference in jump table.
>
> H.J. Lu (2):
> x86: Add a pass to fold tail call
> x86: Fold sibcall targets into jump table
>
> gcc/config/i386/i386-features.cc | 274 +++++++++++++++++++++
> gcc/config/i386/i386-passes.def | 1 +
> gcc/config/i386/i386-protos.h | 3 +
> gcc/config/i386/i386.cc | 12 +
> gcc/config/i386/i386.md | 57 ++++-
> gcc/config/i386/predicates.md | 4 +
> gcc/dwarf2cfi.cc | 7 +-
> gcc/final.cc | 26 +-
> gcc/testsuite/gcc.target/i386/pr14721-1a.c | 54 ++++
> gcc/testsuite/gcc.target/i386/pr14721-1b.c | 37 +++
> gcc/testsuite/gcc.target/i386/pr14721-1c.c | 37 +++
> gcc/testsuite/gcc.target/i386/pr14721-2a.c | 58 +++++
> gcc/testsuite/gcc.target/i386/pr14721-2b.c | 41 +++
> gcc/testsuite/gcc.target/i386/pr14721-2c.c | 43 ++++
> gcc/testsuite/gcc.target/i386/pr14721-3a.c | 56 +++++
> gcc/testsuite/gcc.target/i386/pr14721-3b.c | 40 +++
> gcc/testsuite/gcc.target/i386/pr14721-3c.c | 39 +++
> gcc/testsuite/gcc.target/i386/pr47253-1a.c | 24 ++
> gcc/testsuite/gcc.target/i386/pr47253-1b.c | 17 ++
> gcc/testsuite/gcc.target/i386/pr47253-2a.c | 27 ++
> gcc/testsuite/gcc.target/i386/pr47253-2b.c | 17 ++
> gcc/testsuite/gcc.target/i386/pr47253-3a.c | 32 +++
> gcc/testsuite/gcc.target/i386/pr47253-3b.c | 20 ++
> gcc/testsuite/gcc.target/i386/pr47253-3c.c | 20 ++
> gcc/testsuite/gcc.target/i386/pr47253-4a.c | 26 ++
> gcc/testsuite/gcc.target/i386/pr47253-4b.c | 18 ++
> gcc/testsuite/gcc.target/i386/pr47253-5.c | 15 ++
> gcc/testsuite/gcc.target/i386/pr47253-6.c | 15 ++
> gcc/testsuite/gcc.target/i386/pr47253-7a.c | 52 ++++
> gcc/testsuite/gcc.target/i386/pr47253-7b.c | 36 +++
> 30 files changed, 1097 insertions(+), 11 deletions(-)
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-1a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-1b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-1c.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-2a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-2b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-2c.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-3a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-3b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr14721-3c.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-1a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-1b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-2a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-2b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-3a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-3b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-3c.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-4a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-4b.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-5.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-6.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-7a.c
> create mode 100644 gcc/testsuite/gcc.target/i386/pr47253-7b.c
>
> --
> 2.48.1
>
