[Compilation] Change BOOL_SWITCH to fix Windows compilation

Follow xFormers's DISTPATCH_BOOL. Haven't tested it on Windows.

csrc/flash_attn/src/fmha_bwd_hdim128.cu

@@ -5,7 +5,7 @@
 #include "fmha_bwd_launch_template.h"
 
 void run_fmha_bwd_hdim128(FMHA_dgrad_params &params, cudaStream_t stream, const bool configure) {
-    FP16_SWITCH(params.is_bf16, ({
+    FP16_SWITCH(params.is_bf16, ([&] {
         using Kernel_traits = FMHA_kernel_traits<128, 128, 16, 1, 8, 0x100u, elem_type>;
         run_fmha_bwd_loop<Kernel_traits>(params, stream, configure);
     }));

csrc/flash_attn/src/fmha_bwd_hdim32.cu

@@ -5,7 +5,7 @@
 #include "fmha_bwd_launch_template.h"
 
 void run_fmha_bwd_hdim32(FMHA_dgrad_params &params, cudaStream_t stream, const bool configure) {
-    FP16_SWITCH(params.is_bf16, ({
+    FP16_SWITCH(params.is_bf16, ([&] {
         if (params.seqlen_k == 128) {
             using Kernel_traits = FMHA_kernel_traits<128, 32, 16, 1, 8, 0x08u, elem_type>;
             run_fmha_bwd_loop<Kernel_traits>(params, stream, configure);

csrc/flash_attn/src/fmha_bwd_hdim64.cu

@@ -5,7 +5,7 @@
 #include "fmha_bwd_launch_template.h"
 
 void run_fmha_bwd_hdim64(FMHA_dgrad_params &params, cudaStream_t stream, const bool configure) {
-    FP16_SWITCH(params.is_bf16, ({
+    FP16_SWITCH(params.is_bf16, ([&] {
         auto dprops = at::cuda::getCurrentDeviceProperties();
         if (params.seqlen_k == 128) {
             using Kernel_traits = FMHA_kernel_traits<128, 64, 16, 1, 8, 0x08u, elem_type>;

csrc/flash_attn/src/fmha_bwd_launch_template.h

@@ -61,7 +61,7 @@ void run_fmha_bwd_loop(FMHA_dgrad_params &params, cudaStream_t stream, const boo
 
     bool is_dropout = params.p_dropout < 1.f;  // params.p_dropout is the probability of "keeping"
     // Work-around for gcc 7. It doesn't like nested BOOL_SWITCH.
-    BOOL_SWITCH(is_dropout, IsDropoutConst, ({
+    BOOL_SWITCH(is_dropout, IsDropoutConst, ([&] {
         auto kernel = params.is_causal
             ? &fmha_bwd_dq_dk_dv_loop_kernel<Kernel_traits, IsDropoutConst, true>
             : &fmha_bwd_dq_dk_dv_loop_kernel<Kernel_traits, IsDropoutConst, false>;

csrc/flash_attn/src/fmha_fwd_hdim128.cu

@@ -5,7 +5,7 @@
 #include "fmha_fwd_launch_template.h"
 
 void run_fmha_fwd_hdim128(Launch_params<FMHA_fprop_params> &launch_params) {
-    FP16_SWITCH(launch_params.params.is_bf16, ({
+    FP16_SWITCH(launch_params.params.is_bf16, ([&] {
         using Kernel_traits = FMHA_kernel_traits<128, 128, 16, 1, 4, 0x08u, elem_type>;
         run_fmha_fwd_loop<Kernel_traits>(launch_params);
     }));

csrc/flash_attn/src/fmha_fwd_hdim32.cu

@@ -5,7 +5,7 @@
 #include "fmha_fwd_launch_template.h"
 
 void run_fmha_fwd_hdim32(Launch_params<FMHA_fprop_params> &launch_params) {
-    FP16_SWITCH(launch_params.params.is_bf16, ({
+    FP16_SWITCH(launch_params.params.is_bf16, ([&] {
         if (launch_params.params.seqlen_k == 128) {
             using Kernel_traits = FMHA_kernel_traits<128, 32, 16, 1, 4, 0x08u, elem_type>;
             run_fmha_fwd_loop<Kernel_traits>(launch_params);

csrc/flash_attn/src/fmha_fwd_hdim64.cu

@@ -5,7 +5,7 @@
 #include "fmha_fwd_launch_template.h"
 
 void run_fmha_fwd_hdim64(Launch_params<FMHA_fprop_params> &launch_params) {
-    FP16_SWITCH(launch_params.params.is_bf16, ({
+    FP16_SWITCH(launch_params.params.is_bf16, ([&] {
         if (launch_params.params.seqlen_k == 128) {
             using Kernel_traits = FMHA_kernel_traits<128, 64, 16, 1, 4, 0x08u, elem_type>;
             run_fmha_fwd_loop<Kernel_traits>(launch_params);

csrc/flash_attn/src/fmha_fwd_launch_template.h

@@ -56,7 +56,7 @@ void run_fmha_fwd_loop(Launch_params<FMHA_fprop_params> &launch_params) {
     // Work-around for gcc 7. It doesn't like nested BOOL_SWITCH.
     // https://github.com/kokkos/kokkos-kernels/issues/349
     // https://github.com/HazyResearch/flash-attention/issues/21
-    BOOL_SWITCH(launch_params.is_dropout, IsDropoutConst, ({
+    BOOL_SWITCH(launch_params.is_dropout, IsDropoutConst, ([&] {
         auto kernel = launch_params.params.is_causal
             ? (launch_params.return_softmax
                ? &fmha_fwd_loop_kernel<Kernel_traits, IsDropoutConst, true, true>

csrc/flash_attn/src/static_switch.h

 // Inspired by https://github.com/NVIDIA/DALI/blob/main/include/dali/core/static_switch.h
 // and https://github.com/pytorch/pytorch/blob/master/aten/src/ATen/Dispatch.h
+// and https://github.com/facebookresearch/xformers/blob/main/xformers/csrc/attention/cuda/fmha/gemm_kernel_utils.h#L8
 
 #pragma once
 
@@ -9,27 +10,31 @@
 ///
 /// Usage:
 /// ```
-/// BOOL_SWITCH(flag, BoolConst, ({
+/// BOOL_SWITCH(flag, BoolConst, ([&] {
 ///     some_function<BoolConst>(...);
 /// }));
 /// ```
 /// We need "({" and "})" to make sure that the code is a single argument being passed to the macro.
-#define BOOL_SWITCH(COND, CONST_NAME, CODE) \
-    if (COND) {                             \
-        constexpr bool CONST_NAME = true;   \
-        CODE;                               \
-    } else {                                \
-        constexpr bool CONST_NAME = false;  \
-        CODE;                               \
+#define BOOL_SWITCH(COND, CONST_NAME, F)       \
+    {                                          \
+        if (COND) {                            \
+            constexpr bool CONST_NAME = true;  \
+            F();                               \
+        } else {                               \
+            constexpr bool CONST_NAME = false; \
+            F();                               \
+        }                                      \
     }
 
 // modified from BOOL_SWITCH
 // because MSVC cannot handle std::conditional with constexpr variable
-#define FP16_SWITCH(COND, CODE)        \
-    if (COND) {                        \
-      using elem_type = __nv_bfloat16; \
-      CODE;                            \
-    } else {                           \
-      using elem_type = __half;        \
-      CODE;                            \
-    }                                  \
+#define FP16_SWITCH(COND, F)                 \
+    {                                        \
+        if (COND) {                          \
+            using elem_type = __nv_bfloat16; \
+            F();                             \
+        } else {                             \
+            using elem_type = __half;        \
+            F();                             \
+        }                                    \
+    }