Messages in this thread | | | Date | Tue, 16 Jan 2018 23:07:34 -0500 (EST) | From | Nicolas Pitre <> | Subject | Re: [PATCH] ARM: make memzero optimization smarter |
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On Tue, 16 Jan 2018, Arnd Bergmann wrote:
> On Tue, Jan 16, 2018 at 6:10 PM, Nicolas Pitre <nicolas.pitre@linaro.org> wrote: > > On Tue, 16 Jan 2018, Arnd Bergmann wrote: > > > >> However, we can avoid this class of bogus warnings for the memset() macro > >> by only doing the micro-optimization for zero-length arguments when the > >> length is a compile-time constant. This should also reduce code size by > >> a few bytes, and avoid an extra branch for the cases that a variable-length > >> argument is always nonzero, which is probably the common case anyway. > >> > >> I have made sure that the __memzero implementation can safely handle > >> a zero length argument. > > > > Why not simply drop the test on (__n) != 0 then? I fail to see what the > > advantage is in that case. > > Good point. We might actually get even better results by dropping the > __memzero path entirely, since gcc has can optimize trivial memset() > operations and inline them. > > If I read arch/arm/lib/memzero.S correctly, it saves exactly two 'orr' > instructions compared to the memset.S implementation, but calling > memset() rather than __memzero() from C code ends up saving a > function call at least some of the time. > > Building a defconfig kernel with gcc-7.2.1, I see 1919 calls to __memzero() > and 636 calls to memset() in vmlinux. If I remove the macro entirely, > I get 1775 calls to memset() instead, so 780 memzero instances got > inlined, and kernel shrinks by 5488 bytes (0.03%), not counting the > __memzero implementation that we could possibly also drop.
I get 3668 fewer bytes just by removing the test against 0 in the macro.
And an additional 5092 fewer bytes by removing the call-to-__memzero optimization.
That's using gcc v6.3.1.
> FWIW, the zero-length check saves five references to __memzero() > and one reference to memset(), or 16 bytes in kernel size, I have not > checked what those are.
They apparently are:
security/keys/key.c:1117:2: memset(&ktype->lock_class, 0, sizeof(ktype->lock_class)); crypto/drbg.c:615:3: memset(drbg->V, 1, drbg_statelen(drbg)); crypto/drbg.c:1120:3: memset(drbg->V, 0, drbg_statelen(drbg)); crypto/drbg.c:1121:3: memset(drbg->C, 0, drbg_statelen(drbg)); drivers/crypto/bcm/cipher.c:1963:2: memset(ctx->bcm_spu_req_hdr, 0, alloc_len); drivers/media/platform/vivid/vivid-vbi-cap.c:106:2: memset(vbuf, 0x10, vb2_plane_size(&buf->vb.vb2_buf, 0)); drivers/media/platform/vivid/vivid-vbi-cap.c:127:2: memset(vbuf, 0, vb2_plane_size(&buf->vb.vb2_buf, 0)); drivers/gpu/drm/nouveau/nvkm/subdev/bios/conn.c:50:2: memset(info, 0x00, sizeof(*info));
Nicolas
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