Add small customization to the build (#768)

* Add ability to override the compiler used by NVCC from an environment variable

* Allow relative paths in CANDLE_FLASH_ATTN_BUILD_DIR

* Add the compilation failure to the readme, with a possible solution

* Adjust the error message, and remove the special handling of the relative paths

README.md

@@ -243,6 +243,17 @@ authentication token. See issue
 git submodule update --init
 ```
 
+#### Compiling with flash-attention fails
+
+```
+/usr/include/c++/11/bits/std_function.h:530:146: error: parameter packs not expanded with ‘...’:
+```
+
+This is a bug in gcc-11 triggered by the Cuda compiler. To fix this, install a different, supported gcc version - for example gcc-10, and specify the path to the compiler in the CANDLE_NVCC_CCBIN environment variable.
+```
+env CANDLE_NVCC_CCBIN=/usr/lib/gcc/x86_64-linux-gnu/10 cargo ...
+```
+
 #### Tracking down errors
 
 You can set `RUST_BACKTRACE=1` to be provided with backtraces when a candle

candle-flash-attn/build.rs

@@ -57,9 +57,17 @@ fn main() -> Result<()> {
             #[allow(clippy::redundant_clone)]
             out_dir.clone()
         }
-        Ok(build_dir) => PathBuf::from(build_dir),
+        Ok(build_dir) =>
+        {
+            let path = PathBuf::from(build_dir);
+            path.canonicalize().expect(&format!("Directory doesn't exists: {} (the current directory is {})", &path.display(), std::env::current_dir()?.display()))
+        }
     };
     set_cuda_include_dir()?;
+
+    let ccbin_env = std::env::var("CANDLE_NVCC_CCBIN");
+    println!("cargo:rerun-if-env-changed=CANDLE_NVCC_CCBIN");
+
     let compute_cap = compute_cap()?;
 
     let out_file = build_dir.join("libflashattention.a");
@@ -95,14 +103,21 @@ fn main() -> Result<()> {
                     .args(["--default-stream", "per-thread"])
                     .arg("-Icutlass/include")
                     .arg("--expt-relaxed-constexpr")
-                    .arg(cu_file);
+                    .arg("--verbose");
+                if let Ok(ccbin_path) = &ccbin_env {
+                    command
+                        .arg("-allow-unsupported-compiler")
+                        .args(["-ccbin", ccbin_path]);
+                }
+                command.arg(cu_file);
                 let output = command
                     .spawn()
                     .context("failed spawning nvcc")?
                     .wait_with_output()?;
                 if !output.status.success() {
                     anyhow::bail!(
-                        "nvcc error while compiling:\n\n# stdout\n{:#}\n\n# stderr\n{:#}",
+                        "nvcc error while executing compiling: {:?}\n\n# stdout\n{:#}\n\n# stderr\n{:#}",
+                        &command,
                         String::from_utf8_lossy(&output.stdout),
                         String::from_utf8_lossy(&output.stderr)
                     )
@@ -122,7 +137,8 @@ fn main() -> Result<()> {
             .wait_with_output()?;
         if !output.status.success() {
             anyhow::bail!(
-                "nvcc error while linking:\n\n# stdout\n{:#}\n\n# stderr\n{:#}",
+                "nvcc error while linking: {:?}\n\n# stdout\n{:#}\n\n# stderr\n{:#}",
+                &command,
                 String::from_utf8_lossy(&output.stdout),
                 String::from_utf8_lossy(&output.stderr)
             )