From: Noah Goldstein <goldstein.w.n@gmail.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: libc-alpha@sourceware.org, carlos@systemhalted.org
Subject: Re: [PATCH v1 3/3] x86: Use VMM API in memcmpeq-evex.S and minor changes
Date: Mon, 31 Oct 2022 11:42:19 -0500 [thread overview]
Message-ID: <CAFUsyfJJFmeX9yOcK9gpHAN0Dt6vPHNGecBu5R9o-6RaXAFsyQ@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOq4uiXPr2m6A8822OhD=0gXnoowpABLGkqPp8YY81=WSw@mail.gmail.com>
On Mon, Oct 31, 2022 at 10:48 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Sat, Oct 29, 2022 at 1:20 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > Changes to generated code are:
> > 1. In a few places use `vpcmpeqb` instead of `vpcmpneq` to save a
> > byte of code size.
> > 2. Add a branch for length <= (VEC_SIZE * 6) as opposed to doing
> > the entire block of [VEC_SIZE * 4 + 1, VEC_SIZE * 8] in a
> > single basic-block (the space to add the extra branch without
> > changing code size is bought with the above change).
> >
> > Change (2) has roughly a 20-25% speedup for sizes in [VEC_SIZE * 4 +
> > 1, VEC_SIZE * 6] and negligible to no-cost for [VEC_SIZE * 6 + 1,
> > VEC_SIZE * 8]
> >
> > From N=10 runs on Tigerlake:
> >
> > align1,align2 ,length ,result ,New TIme ,Cur Time,New Time / Old Time
> > 0 ,0 ,129 ,0 ,5.404 ,6.887 ,0.785
> > 0 ,0 ,129 ,1 ,5.308 ,6.826 ,0.778
> > 0 ,0 ,129 ,18446744073709551615 ,5.359 ,6.823 ,0.785
> > 0 ,0 ,161 ,0 ,5.284 ,6.827 ,0.774
> > 0 ,0 ,161 ,1 ,5.317 ,6.745 ,0.788
> > 0 ,0 ,161 ,18446744073709551615 ,5.406 ,6.778 ,0.798
> >
> > 0 ,0 ,193 ,0 ,6.969 ,6.832 ,1.000
> > 0 ,0 ,193 ,1 ,6.943 ,6.748 ,1.029
> > 0 ,0 ,193 ,18446744073709551615 ,6.997 ,6.728 ,1.011
> > 0 ,0 ,225 ,0 ,7.144 ,6.746 ,0.989
> > 0 ,0 ,225 ,1 ,7.218 ,6.683 ,1.003
> > 0 ,0 ,225 ,18446744073709551615 ,6.864 ,6.767 ,0.992
> > 0 ,0 ,256 ,0 ,5.423 ,5.482 ,0.989
> > 0 ,0 ,256 ,1 ,5.348 ,5.465 ,0.978
> > 0 ,0 ,256 ,18446744073709551615 ,5.321 ,5.518 ,0.964
> >
> > Rewriting with VMM API allows for memcmpeq-evex to be used with
> > evex512 by including "x86-evex512-vecs.h" at the top.
> >
> > Complete check passes on x86-64.
> > ---
> > sysdeps/x86_64/multiarch/memcmpeq-evex.S | 255 ++++++++++++++---------
> > 1 file changed, 155 insertions(+), 100 deletions(-)
> >
> > diff --git a/sysdeps/x86_64/multiarch/memcmpeq-evex.S b/sysdeps/x86_64/multiarch/memcmpeq-evex.S
> > index 41124ef1d3..671d19393e 100644
> > --- a/sysdeps/x86_64/multiarch/memcmpeq-evex.S
> > +++ b/sysdeps/x86_64/multiarch/memcmpeq-evex.S
> > @@ -41,24 +41,53 @@
> > # define MEMCMPEQ __memcmpeq_evex
> > # endif
> >
> > +# ifndef VEC_SIZE
> > +# include "x86-evex512-vecs.h"
> > +# endif
> > +# include "reg-macros.h"
> > +
> > +
> > +# if VEC_SIZE == 32
> > +
> > +# define TEST_ZERO_VCMP(reg) inc %VGPR(reg)
> > +# define TEST_ZERO(reg) test %VGPR(reg), %VGPR(reg)
> > +
> > +# define TO_32BIT_P1(reg) /* Do nothing. */
> > +# define TO_32BIT_P2(reg) /* Do nothing. */
> > +# define TO_32BIT(reg) /* Do nothing. */
> > +
> > +# define VEC_CMP VPCMPEQ
> > +
> > +# elif VEC_SIZE == 64
> > +
> > +# define TEST_ZERO_VCMP(reg) TEST_ZERO(reg)
> > +# define TEST_ZERO(reg) neg %VGPR(reg)
> > +
> > +
> > + /* VEC_SIZE == 64 needs to reduce the 64-bit mask to a 32-bit
> > + int. We have two methods for this. If the mask with branched
> > + on, we use `neg` for the branch then `sbb` to get the 32-bit
> > + return. If the mask was no branched on, we just use
> > + `popcntq`. */
> > +# define TO_32BIT_P1(reg) TEST_ZERO(reg)
> > +# define TO_32BIT_P2(reg) sbb %VGPR_SZ(reg, 32), %VGPR_SZ(reg, 32)
> > +# define TO_32BIT(reg) popcntq %reg, %reg
> > +
> > +# define VEC_CMP VPCMPNEQ
> > +
> > +# else
> > +# error "Unsupported VEC_SIZE"
> > +# endif
> > +
> > +
> > # define VMOVU_MASK vmovdqu8
> > -# define VMOVU vmovdqu64
> > -# define VPCMP vpcmpub
> > +# define VPCMPNEQ vpcmpneqb
> > +# define VPCMPEQ vpcmpeqb
> > # define VPTEST vptestmb
> >
> > -# define VEC_SIZE 32
> > # define PAGE_SIZE 4096
> >
> > -# define YMM0 ymm16
> > -# define YMM1 ymm17
> > -# define YMM2 ymm18
> > -# define YMM3 ymm19
> > -# define YMM4 ymm20
> > -# define YMM5 ymm21
> > -# define YMM6 ymm22
> > -
> > -
> > - .section .text.evex, "ax", @progbits
> > + .section SECTION(.text), "ax", @progbits
> > ENTRY_P2ALIGN (MEMCMPEQ, 6)
> > # ifdef __ILP32__
> > /* Clear the upper 32 bits. */
> > @@ -69,47 +98,54 @@ ENTRY_P2ALIGN (MEMCMPEQ, 6)
> > ja L(more_1x_vec)
> >
> > /* Create mask of bytes that are guranteed to be valid because
> > - of length (edx). Using masked movs allows us to skip checks for
> > - page crosses/zero size. */
> > - movl $-1, %ecx
> > - bzhil %edx, %ecx, %ecx
> > - kmovd %ecx, %k2
> > + of length (edx). Using masked movs allows us to skip checks
> > + for page crosses/zero size. */
> > + mov $-1, %VRAX
> > + bzhi %VRDX, %VRAX, %VRAX
> > + /* NB: A `jz` might be useful here. Page-faults that are
> > + invalidated by predicate execution (the evex mask) can be
> > + very slow. The expectation is this is not the norm so and
> > + "most" code will not regularly call 'memcmp' with length = 0
> > + and memory that is not wired up. */
> > + KMOV %VRAX, %k2
> >
> > /* Use masked loads as VEC_SIZE could page cross where length
> > (edx) would not. */
> > - VMOVU_MASK (%rsi), %YMM2{%k2}
> > - VPCMP $4,(%rdi), %YMM2, %k1{%k2}
> > - kmovd %k1, %eax
> > + VMOVU_MASK (%rsi), %VMM(2){%k2}{z}
> > + VPCMPNEQ (%rdi), %VMM(2), %k1{%k2}
> > + KMOV %k1, %VRAX
> > + TO_32BIT (VRAX)
> > ret
> >
> > -
> > + .p2align 4,, 3
> > L(last_1x_vec):
> > - VMOVU -(VEC_SIZE * 1)(%rsi, %rdx), %YMM1
> > - VPCMP $4, -(VEC_SIZE * 1)(%rdi, %rdx), %YMM1, %k1
> > - kmovd %k1, %eax
> > + VMOVU -(VEC_SIZE * 1)(%rsi, %rdx), %VMM(1)
> > + VPCMPNEQ -(VEC_SIZE * 1)(%rdi, %rdx), %VMM(1), %k1
> > + KMOV %k1, %VRAX
> > + TO_32BIT_P1 (rax)
> > L(return_neq0):
> > + TO_32BIT_P2 (rax)
> > ret
> >
> >
> > -
> > - .p2align 4
> > + .p2align 4,, 12
> > L(more_1x_vec):
> > /* From VEC + 1 to 2 * VEC. */
> > - VMOVU (%rsi), %YMM1
> > + VMOVU (%rsi), %VMM(1)
> > /* Use compare not equals to directly check for mismatch. */
> > - VPCMP $4,(%rdi), %YMM1, %k1
> > - kmovd %k1, %eax
> > - testl %eax, %eax
> > + VPCMPNEQ (%rdi), %VMM(1), %k1
> > + KMOV %k1, %VRAX
> > + TEST_ZERO (rax)
> > jnz L(return_neq0)
> >
> > cmpq $(VEC_SIZE * 2), %rdx
> > jbe L(last_1x_vec)
> >
> > /* Check second VEC no matter what. */
> > - VMOVU VEC_SIZE(%rsi), %YMM2
> > - VPCMP $4, VEC_SIZE(%rdi), %YMM2, %k1
> > - kmovd %k1, %eax
> > - testl %eax, %eax
> > + VMOVU VEC_SIZE(%rsi), %VMM(2)
> > + VPCMPNEQ VEC_SIZE(%rdi), %VMM(2), %k1
> > + KMOV %k1, %VRAX
> > + TEST_ZERO (rax)
> > jnz L(return_neq0)
> >
> > /* Less than 4 * VEC. */
> > @@ -117,16 +153,16 @@ L(more_1x_vec):
> > jbe L(last_2x_vec)
> >
> > /* Check third and fourth VEC no matter what. */
> > - VMOVU (VEC_SIZE * 2)(%rsi), %YMM3
> > - VPCMP $4,(VEC_SIZE * 2)(%rdi), %YMM3, %k1
> > - kmovd %k1, %eax
> > - testl %eax, %eax
> > + VMOVU (VEC_SIZE * 2)(%rsi), %VMM(3)
> > + VEC_CMP (VEC_SIZE * 2)(%rdi), %VMM(3), %k1
> > + KMOV %k1, %VRAX
> > + TEST_ZERO_VCMP (rax)
> > jnz L(return_neq0)
> >
> > - VMOVU (VEC_SIZE * 3)(%rsi), %YMM4
> > - VPCMP $4,(VEC_SIZE * 3)(%rdi), %YMM4, %k1
> > - kmovd %k1, %eax
> > - testl %eax, %eax
> > + VMOVU (VEC_SIZE * 3)(%rsi), %VMM(4)
> > + VEC_CMP (VEC_SIZE * 3)(%rdi), %VMM(4), %k1
> > + KMOV %k1, %VRAX
> > + TEST_ZERO_VCMP (rax)
> > jnz L(return_neq0)
> >
> > /* Go to 4x VEC loop. */
> > @@ -136,8 +172,8 @@ L(more_1x_vec):
> > /* Handle remainder of size = 4 * VEC + 1 to 8 * VEC without any
> > branches. */
> >
> > - VMOVU -(VEC_SIZE * 4)(%rsi, %rdx), %YMM1
> > - VMOVU -(VEC_SIZE * 3)(%rsi, %rdx), %YMM2
> > + VMOVU -(VEC_SIZE * 1)(%rsi, %rdx), %VMM(1)
> > + VMOVU -(VEC_SIZE * 2)(%rsi, %rdx), %VMM(2)
> > addq %rdx, %rdi
> >
> > /* Wait to load from s1 until addressed adjust due to
> > @@ -145,26 +181,32 @@ L(more_1x_vec):
> >
> > /* vpxor will be all 0s if s1 and s2 are equal. Otherwise it
> > will have some 1s. */
> > - vpxorq -(VEC_SIZE * 4)(%rdi), %YMM1, %YMM1
> > - /* Ternary logic to xor -(VEC_SIZE * 3)(%rdi) with YMM2 while
> > - oring with YMM1. Result is stored in YMM1. */
> > - vpternlogd $0xde, -(VEC_SIZE * 3)(%rdi), %YMM1, %YMM2
> > -
> > - VMOVU -(VEC_SIZE * 2)(%rsi, %rdx), %YMM3
> > - vpxorq -(VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> > - /* Or together YMM1, YMM2, and YMM3 into YMM3. */
> > - VMOVU -(VEC_SIZE)(%rsi, %rdx), %YMM4
> > - vpxorq -(VEC_SIZE)(%rdi), %YMM4, %YMM4
> > -
> > - /* Or together YMM2, YMM3, and YMM4 into YMM4. */
> > - vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > -
> > - /* Compare YMM4 with 0. If any 1s s1 and s2 don't match. */
> > - VPTEST %YMM4, %YMM4, %k1
> > - kmovd %k1, %eax
> > + vpxorq -(VEC_SIZE * 1)(%rdi), %VMM(1), %VMM(1)
> > + /* Ternary logic to xor -(VEC_SIZE * 3)(%rdi) with VEC(2) while
> > + oring with VEC(1). Result is stored in VEC(1). */
> > + vpternlogd $0xde, -(VEC_SIZE * 2)(%rdi), %VMM(1), %VMM(2)
> > +
> > + cmpl $(VEC_SIZE * 6), %edx
> > + jbe L(4x_last_2x_vec)
> > +
> > + VMOVU -(VEC_SIZE * 3)(%rsi, %rdx), %VMM(3)
> > + vpxorq -(VEC_SIZE * 3)(%rdi), %VMM(3), %VMM(3)
> > + /* Or together VEC(1), VEC(2), and VEC(3) into VEC(3). */
> > + VMOVU -(VEC_SIZE * 4)(%rsi, %rdx), %VMM(4)
> > + vpxorq -(VEC_SIZE * 4)(%rdi), %VMM(4), %VMM(4)
> > +
> > + /* Or together VEC(4), VEC(3), and VEC(2) into VEC(2). */
> > + vpternlogd $0xfe, %VMM(4), %VMM(3), %VMM(2)
> > +
> > + /* Compare VEC(4) with 0. If any 1s s1 and s2 don't match. */
> > +L(4x_last_2x_vec):
> > + VPTEST %VMM(2), %VMM(2), %k1
> > + KMOV %k1, %VRAX
> > + TO_32BIT (VRAX)
> > ret
> >
> > - .p2align 4
> > +
> > + .p2align 4,, 10
> > L(more_8x_vec):
> > /* Set end of s1 in rdx. */
> > leaq -(VEC_SIZE * 4)(%rdi, %rdx), %rdx
> > @@ -175,67 +217,80 @@ L(more_8x_vec):
> > andq $-VEC_SIZE, %rdi
> > /* Adjust because first 4x vec where check already. */
> > subq $-(VEC_SIZE * 4), %rdi
> > - .p2align 4
> > + .p2align 5,, 12
> > + .p2align 4,, 8
> > L(loop_4x_vec):
> > - VMOVU (%rsi, %rdi), %YMM1
> > - vpxorq (%rdi), %YMM1, %YMM1
> > + VMOVU (%rsi, %rdi), %VMM(1)
> > + vpxorq (%rdi), %VMM(1), %VMM(1)
> >
> > - VMOVU VEC_SIZE(%rsi, %rdi), %YMM2
> > - vpternlogd $0xde,(VEC_SIZE)(%rdi), %YMM1, %YMM2
> > + VMOVU VEC_SIZE(%rsi, %rdi), %VMM(2)
> > + vpternlogd $0xde, (VEC_SIZE)(%rdi), %VMM(1), %VMM(2)
> >
> > - VMOVU (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
> > - vpxorq (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> > + VMOVU (VEC_SIZE * 2)(%rsi, %rdi), %VMM(3)
> > + vpxorq (VEC_SIZE * 2)(%rdi), %VMM(3), %VMM(3)
> >
> > - VMOVU (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
> > - vpxorq (VEC_SIZE * 3)(%rdi), %YMM4, %YMM4
> > + VMOVU (VEC_SIZE * 3)(%rsi, %rdi), %VMM(4)
> > + vpxorq (VEC_SIZE * 3)(%rdi), %VMM(4), %VMM(4)
> >
> > - vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > - VPTEST %YMM4, %YMM4, %k1
> > - kmovd %k1, %eax
> > - testl %eax, %eax
> > + vpternlogd $0xfe, %VMM(2), %VMM(3), %VMM(4)
> > + VPTEST %VMM(4), %VMM(4), %k1
> > + KMOV %k1, %VRAX
> > + TEST_ZERO (rax)
> > jnz L(return_neq2)
> > subq $-(VEC_SIZE * 4), %rdi
> > cmpq %rdx, %rdi
> > jb L(loop_4x_vec)
> >
> > subq %rdx, %rdi
> > - VMOVU (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
> > - vpxorq (VEC_SIZE * 3)(%rdx), %YMM4, %YMM4
> > +
> > + VMOVU (VEC_SIZE * 3)(%rsi, %rdx), %VMM(4)
> > + vpxorq (VEC_SIZE * 3)(%rdx), %VMM(4), %VMM(4)
> > /* rdi has 4 * VEC_SIZE - remaining length. */
> > - cmpl $(VEC_SIZE * 3), %edi
> > - jae L(8x_last_1x_vec)
> > +
> > /* Load regardless of branch. */
> > - VMOVU (VEC_SIZE * 2)(%rsi, %rdx), %YMM3
> > - /* Ternary logic to xor (VEC_SIZE * 2)(%rdx) with YMM3 while
> > - oring with YMM4. Result is stored in YMM4. */
> > - vpternlogd $0xf6,(VEC_SIZE * 2)(%rdx), %YMM3, %YMM4
> > + VMOVU (VEC_SIZE * 2)(%rsi, %rdx), %VMM(3)
> > + /* Ternary logic to xor (VEC_SIZE * 2)(%rdx) with VEC(3) while
> > + oring with VEC(4). Result is stored in VEC(4). */
> > + vpternlogd $0xf6, (VEC_SIZE * 2)(%rdx), %VMM(3), %VMM(4)
> > +
> > + /* Seperate logic as we can only use testb for VEC_SIZE == 64.
> > + */
> > +# if VEC_SIZE == 64
> > + testb %dil, %dil
> > + js L(8x_last_2x_vec)
> > +# else
> > cmpl $(VEC_SIZE * 2), %edi
> > - jae L(8x_last_2x_vec)
> > + jge L(8x_last_2x_vec)
> > +# endif
> >
> > - VMOVU VEC_SIZE(%rsi, %rdx), %YMM2
> > - vpxorq VEC_SIZE(%rdx), %YMM2, %YMM2
> > + VMOVU VEC_SIZE(%rsi, %rdx), %VMM(2)
> > + vpxorq VEC_SIZE(%rdx), %VMM(2), %VMM(2)
> >
> > - VMOVU (%rsi, %rdx), %YMM1
> > - vpxorq (%rdx), %YMM1, %YMM1
> > + VMOVU (%rsi, %rdx), %VMM(1)
> > + vpxorq (%rdx), %VMM(1), %VMM(1)
> >
> > - vpternlogd $0xfe, %YMM1, %YMM2, %YMM4
> > + vpternlogd $0xfe, %VMM(1), %VMM(2), %VMM(4)
> > L(8x_last_1x_vec):
> > L(8x_last_2x_vec):
> > - VPTEST %YMM4, %YMM4, %k1
> > - kmovd %k1, %eax
> > + VPTEST %VMM(4), %VMM(4), %k1
> > + KMOV %k1, %VRAX
> > + TO_32BIT_P1 (rax)
> > L(return_neq2):
> > + TO_32BIT_P2 (rax)
> > ret
> >
> > - .p2align 4,, 8
> > + .p2align 4,, 4
> > L(last_2x_vec):
> > - VMOVU -(VEC_SIZE * 2)(%rsi, %rdx), %YMM1
> > - vpxorq -(VEC_SIZE * 2)(%rdi, %rdx), %YMM1, %YMM1
> > - VMOVU -(VEC_SIZE * 1)(%rsi, %rdx), %YMM2
> > - vpternlogd $0xde, -(VEC_SIZE * 1)(%rdi, %rdx), %YMM1, %YMM2
> > - VPTEST %YMM2, %YMM2, %k1
> > - kmovd %k1, %eax
> > + VMOVU -(VEC_SIZE * 2)(%rsi, %rdx), %VMM(1)
> > + vpxorq -(VEC_SIZE * 2)(%rdi, %rdx), %VMM(1), %VMM(1)
> > + VMOVU -(VEC_SIZE * 1)(%rsi, %rdx), %VMM(2)
> > + vpternlogd $0xde, -(VEC_SIZE * 1)(%rdi, %rdx), %VMM(1), %VMM(2)
> > + VPTEST %VMM(2), %VMM(2), %k1
> > + KMOV %k1, %VRAX
> > + TO_32BIT (VRAX)
> > ret
> >
> > - /* 1 Bytes from next cache line. */
> > + /* evex256: 1 Bytes from next cache line. evex512: 15 Bytes from
> > + next cache line. */
> > END (MEMCMPEQ)
> > #endif
> > --
> > 2.34.1
> >
>
> LGTM.
>
> Thanks.
>
> --
> H.J.
Err realized I had messed up my extraction script when consolidating these
numbers (ratios are correct, had pulled in the wrong fields when grabbing
the actual times).
Correct numbers are:
align1,align2 ,length ,result ,memcmpeq-v2 ,current
,memcmpeq-v2/current
0 ,0 ,129 ,0 ,5.404 ,6.887 ,0.785
0 ,0 ,129 ,1 ,5.308 ,6.826 ,0.778
0 ,0 ,129 ,18446744073709551615 ,5.359 ,6.823 ,0.785
0 ,0 ,161 ,0 ,5.284 ,6.827 ,0.774
0 ,0 ,161 ,1 ,5.317 ,6.745 ,0.788
0 ,0 ,161 ,18446744073709551615 ,5.406 ,6.778 ,0.798
0 ,0 ,193 ,0 ,6.804 ,6.802 ,1.000
0 ,0 ,193 ,1 ,6.950 ,6.754 ,1.029
0 ,0 ,193 ,18446744073709551615 ,6.792 ,6.719 ,1.011
0 ,0 ,225 ,0 ,6.625 ,6.699 ,0.989
0 ,0 ,225 ,1 ,6.776 ,6.735 ,1.003
0 ,0 ,225 ,18446744073709551615 ,6.758 ,6.738 ,0.992
0 ,0 ,256 ,0 ,5.402 ,5.462
,0.989
0 ,0 ,256 ,1 ,5.364 ,5.483
,0.978
0 ,0 ,256 ,18446744073709551615 ,5.341 ,5.539 ,0.964
Will update the commit message before pushing.
next prev parent reply other threads:[~2022-10-31 16:42 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-10-29 20:19 [PATCH v1 1/3] string: Add len=0 to {w}memcmp{eq} tests and benchtests Noah Goldstein
2022-10-29 20:19 ` [PATCH v1 2/3] x86: Use VMM API in memcmp-evex-movbe.S and minor changes Noah Goldstein
2022-10-31 15:47 ` H.J. Lu
2022-10-29 20:19 ` [PATCH v1 3/3] x86: Use VMM API in memcmpeq-evex.S " Noah Goldstein
2022-10-31 15:48 ` H.J. Lu
2022-10-31 16:42 ` Noah Goldstein [this message]
2022-10-31 13:19 ` [PATCH v1 1/3] string: Add len=0 to {w}memcmp{eq} tests and benchtests Siddhesh Poyarekar
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--in-reply-to=CAFUsyfJJFmeX9yOcK9gpHAN0Dt6vPHNGecBu5R9o-6RaXAFsyQ@mail.gmail.com \
--to=goldstein.w.n@gmail.com \
--cc=carlos@systemhalted.org \
--cc=hjl.tools@gmail.com \
--cc=libc-alpha@sourceware.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
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via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
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