[libclc] [libclc] Update build instructions in readme (PR #111369)

[David Spickett via cfe-commits]

https://github.com/DavidSpickett created https://github.com/llvm/llvm-project/pull/111369

The configure Python script was removed by
d6e0e6d255a7d54a3873b7a5d048eee00ef6bb6d / https://reviews.llvm.org/D69966.

The readme was never updated with the cmake way to do it. I couldn't find any dedicated buildbots for this so I'm making an educated guess. This is what built locally for me.

>From a2dd2330359ecd3c04e01b6f9121481e6784014a Mon Sep 17 00:00:00 2001
From: David Spickett <david.spickett at linaro.org>
Date: Mon, 7 Oct 2024 12:29:41 +0000
Subject: [PATCH] [libclc] Update build instructions in readme

The configure Python script was removed by
d6e0e6d255a7d54a3873b7a5d048eee00ef6bb6d / https://reviews.llvm.org/D69966.

The readme was never updated with the cmake way to do it.
I couldn't find any dedicated buildbots for this so I'm making
an educated guess. This is what built locally for me.
---
 libclc/README.TXT | 34 +++++++++++++++++++++++-----------
 1 file changed, 23 insertions(+), 11 deletions(-)

diff --git a/libclc/README.TXT b/libclc/README.TXT
index 57b5242b9bbecb..f64f0e05a742f2 100644
--- a/libclc/README.TXT
+++ b/libclc/README.TXT
@@ -23,29 +23,41 @@ generic implementations of most library requirements, allowing the target
 to override the generic implementation at the granularity of individual
 functions.
 
-libclc currently only supports the PTX target, but support for more
+libclc currently supports NVPTX, AMDGPU and SPIRV targets, but support for more
 targets is welcome.
 
-Compiling and installing with Make
-----------------------------------
+Compiling and installing
+------------------------
 
-$ ./configure.py --with-llvm-config=/path/to/llvm-config && make
-$ make install
+(in the following instructions you can use make or ninja)
 
-Note you can use the DESTDIR Makefile variable to do staged installs.
+For an in-tree build, Clang must also be built at the same time:
 
-$ make install DESTDIR=/path/for/staged/install
+$ cmake <path-to>/llvm-project/llvm/CMakeLists.txt -DLLVM_ENABLE_PROJECTS="libclc;clang" \
+    -DCMAKE_BUILD_TYPE=Release -G Ninja
+$ ninja
 
-Compiling and installing with Ninja
------------------------------------
+Then install:
 
-$ ./configure.py -g ninja --with-llvm-config=/path/to/llvm-config && ninja
 $ ninja install
 
-Note you can use the DESTDIR environment variable to do staged installs.
+Note you can use the DESTDIR Makefile variable to do staged installs.
 
 $ DESTDIR=/path/for/staged/install ninja install
 
+To build against an existing LLVM build or install:
+
+$ cmake <path-to>/llvm-project/libclc/CMakeLists.txt -DCMAKE_BUILD_TYPE=Release \
+  -G Ninja -DLLVM_DIR=$(<path-to>/llvm-config --cmakedir)
+$ ninja
+
+Then install as before.
+
+In both cases, the LLVM used must include the targets you want libclc support for.
+AMDGPU and NVPTX are enabled in LLVM by default, but SPIRV must be enabled by
+passing `-DLLVM_EXPERIMENTAL_TARGETS_TO_BUILD=SPIRV` to CMake when configuring
+LLVM.
+
 Website
 -------