Re: [PATCH v2 1/2] riscv: Add support for strlen inline expansion

[Jeff Law <jeffreyalaw@gmail.com>]

On 9/6/23 10:07, Christoph Muellner wrote:
> From: Christoph Müllner <christoph.muellner@vrull.eu>
> 
> This patch implements the expansion of the strlen builtin for RV32/RV64
> for xlen-aligned aligned strings if Zbb or XTheadBb instructions are available.
> The inserted sequences are:
> 
> rv32gc_zbb (RV64 is similar):
>        add     a3,a0,4
>        li      a4,-1
> .L1:  lw      a5,0(a0)
>        add     a0,a0,4
>        orc.b   a5,a5
>        beq     a5,a4,.L1
>        not     a5,a5
>        ctz     a5,a5
>        srl     a5,a5,0x3
>        add     a0,a0,a5
>        sub     a0,a0,a3
> 
> rv64gc_xtheadbb (RV32 is similar):
>        add       a4,a0,8
> .L2:  ld        a5,0(a0)
>        add       a0,a0,8
>        th.tstnbz a5,a5
>        beqz      a5,.L2
>        th.rev    a5,a5
>        th.ff1    a5,a5
>        srl       a5,a5,0x3
>        add       a0,a0,a5
>        sub       a0,a0,a4
> 
> This allows to inline calls to strlen(), with optimized code for
> xlen-aligned strings, resulting in the following benefits over
> a call to libc:
> * no call/ret instructions
> * no stack frame allocation
> * no register saving/restoring
> * no alignment test
> 
> The inlining mechanism is gated by a new switch ('-minline-strlen')
> and by the variable 'optimize_size'.
> 
> Tested using the glibc string tests.
> 
> Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> 
> gcc/ChangeLog:
> 
> 	* config.gcc: Add new object riscv-string.o.
> 	riscv-string.cc.
> 	* config/riscv/riscv-protos.h (riscv_expand_strlen):
> 	New function.
> 	* config/riscv/riscv.md (strlen<mode>): New expand INSN.
> 	* config/riscv/riscv.opt: New flag 'minline-strlen'.
> 	* config/riscv/t-riscv: Add new object riscv-string.o.
> 	* config/riscv/thead.md (th_rev<mode>2): Export INSN name.
> 	(th_rev<mode>2): Likewise.
> 	(th_tstnbz<mode>2): New INSN.
> 	* doc/invoke.texi: Document '-minline-strlen'.
> 	* emit-rtl.cc (emit_likely_jump_insn): New helper function.
> 	(emit_unlikely_jump_insn): Likewise.
> 	* rtl.h (emit_likely_jump_insn): New prototype.
> 	(emit_unlikely_jump_insn): Likewise.
> 	* config/riscv/riscv-string.cc: New file.
> 
> gcc/testsuite/ChangeLog:
> 
> 	* gcc.target/riscv/xtheadbb-strlen-unaligned.c: New test.
> 	* gcc.target/riscv/xtheadbb-strlen.c: New test.
> 	* gcc.target/riscv/zbb-strlen-disabled-2.c: New test.
> 	* gcc.target/riscv/zbb-strlen-disabled.c: New test.
> 	* gcc.target/riscv/zbb-strlen-unaligned.c: New test.
> 	* gcc.target/riscv/zbb-strlen.c: New test.
Note that I don't think we need the new UNSPEC_STRLEN since its only 
used in the expander and doesn't survive into RTL.  Your call on whether 
or not to remove it now or as a separate patch (or keep it if I'm wrong 
about it not being needed.

OK for the trunk.  Sorry this got lost in the shuffle last year.

jeff
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