Enable im2col on the cpu side. (#805)

* Enable im2col on the cpu side.

* Hook im2col on the cpu backend.

* Use the kernel offset.

* Avoid an unnecessary copy.

* Handle non-contiguous kernels.

* Add a const to select the conv2d kernel.

candle-core/src/cpu_backend.rs

@@ -4,6 +4,8 @@ use crate::{DType, Error, IntDType, Layout, Result, Shape, WithDType};
 use half::{bf16, f16};
 use rayon::prelude::*;
 
+const USE_IM2COL_CONV2D: bool = true;
+
 // TODO: Maybe we should not implement [Clone] here and instead have an explicit allocator +
 // intercept the oom errors to avoid panicking and provide a proper error.
 #[derive(Debug, Clone)]
@@ -1089,6 +1091,81 @@ impl<'a> Map2 for Conv1D<'a> {
     }
 }
 
+struct Im2Col {
+    h_k: usize,
+    w_k: usize,
+    stride: usize,
+    dilation: usize,
+    padding: usize,
+}
+
+impl Im2Col {
+    fn hw_out(&self, h: usize, w: usize) -> (usize, usize) {
+        let h_out = (h + 2 * self.padding - self.dilation * (self.h_k - 1) - 1) / self.stride + 1;
+        let w_out = (w + 2 * self.padding - self.dilation * (self.w_k - 1) - 1) / self.stride + 1;
+        (h_out, w_out)
+    }
+}
+
+impl Map1 for Im2Col {
+    fn f<T: WithDType>(&self, vs: &[T], layout: &Layout) -> Result<Vec<T>> {
+        let &Self {
+            h_k,
+            w_k,
+            stride,
+            dilation,
+            padding,
+        } = self;
+        let (b, c, h, w) = layout.shape().dims4()?;
+        let (h_out, w_out) = self.hw_out(h, w);
+        let src = &vs[layout.start_offset()..];
+        let mut dst = vec![T::zero(); b * h_out * w_out * c * h_k * w_k];
+        let (src_s0, src_s1, src_s2, src_s3) = {
+            let s = layout.stride();
+            (s[0], s[1], s[2], s[3])
+        };
+        // TODO: provide specialized kernels for the common use cases.
+        // - h_k = w_k = 1
+        // - padding = 0
+        // - stride = 1
+        // - dilation = 1
+        for b_idx in 0..b {
+            let src_idx = b_idx * src_s0;
+            let dst_idx = b_idx * h_out * w_out * c * h_k * w_k;
+            for h_idx in 0..h_out {
+                let dst_idx = dst_idx + h_idx * w_out * c * h_k * w_k;
+                for w_idx in 0..w_out {
+                    let dst_idx = dst_idx + w_idx * c * h_k * w_k;
+                    for c_idx in 0..c {
+                        let dst_idx = dst_idx + c_idx * h_k * w_k;
+                        let src_idx = c_idx * src_s1 + src_idx;
+                        for h_k_idx in 0..h_k {
+                            let src_h = h_idx * stride + h_k_idx * dilation;
+                            if padding != 0 && (src_h < padding || src_h >= h + padding) {
+                                continue;
+                            }
+                            let src_h = src_h - padding;
+                            let src_idx = src_idx + src_h * src_s2;
+                            let dst_idx = dst_idx + h_k_idx * w_k;
+                            for w_k_idx in 0..w_k {
+                                let src_w = w_idx * stride + w_k_idx * dilation;
+                                if padding != 0 && (src_w < padding || src_w >= h + padding) {
+                                    continue;
+                                }
+                                let src_w = src_w - padding;
+                                let src_idx = src_idx + src_w * src_s3;
+                                let dst_idx = dst_idx + w_k_idx;
+                                dst[dst_idx] = src[src_idx]
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        Ok(dst)
+    }
+}
+
 struct Conv2D<'a>(&'a crate::conv::ParamsConv2D);
 
 impl<'a> Map2 for Conv2D<'a> {
@@ -2237,7 +2314,43 @@ impl BackendStorage for CpuStorage {
         kernel_l: &Layout,
         params: &crate::conv::ParamsConv2D,
     ) -> Result<Self> {
-        Conv2D(params).map(self, l, kernel, kernel_l)
+        if !USE_IM2COL_CONV2D {
+            return Conv2D(params).map(self, l, kernel, kernel_l);
+        }
+        let op = Im2Col {
+            h_k: params.k_h,
+            w_k: params.k_w,
+            padding: params.padding,
+            stride: params.stride,
+            dilation: params.dilation,
+        };
+        let col = op.map(self, l)?;
+        let b = params.b_size;
+        let n = params.c_out;
+        let (h_out, w_out) = (params.out_h(), params.out_w());
+        let k = op.h_k * op.w_k * params.c_in;
+        let m = h_out * w_out;
+        let col_l = Layout::contiguous((b, m, k));
+        let res = if kernel_l.is_contiguous() {
+            let kernel_l = Layout::contiguous_with_offset((1, n, k), kernel_l.start_offset())
+                .transpose(1, 2)?
+                .broadcast_as((b, k, n))?;
+            col.matmul(kernel, (b, m, n, k), &col_l, &kernel_l)?
+        } else {
+            // Make the kernel contiguous if not already the case.
+            let mut kernel_c = self.device().zeros_impl(kernel_l.shape(), kernel.dtype())?;
+            kernel.copy_strided_src(&mut kernel_c, 0, kernel_l)?;
+            let kernel_l = Layout::contiguous_with_offset((1, n, k), kernel_l.start_offset())
+                .transpose(1, 2)?
+                .broadcast_as((b, k, n))?;
+            col.matmul(kernel, (b, m, n, k), &col_l, &kernel_l)?
+        };
+        let res_l = Layout::contiguous((b, h_out, w_out, params.c_out))
+            .transpose(1, 2)?
+            .transpose(1, 3)?;
+        let mut res_t = self.device().zeros_impl(res_l.shape(), res.dtype())?;
+        res.copy_strided_src(&mut res_t, 0, &res_l)?;
+        Ok(res_t)
     }
 
     fn conv_transpose2d(