[llvm-dev] llvm-exegesis broken on x86

[John Brawn via llvm-dev]

What’s happening is that
  set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} X86" PARENT_SCOPE)
and
set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} AArch64" PARENT_SCOPE)
set LLVM_EXEGESIS_TARGETS in the parent scope and not the local scope, but read from the local scope.
So when the first executes the local LLVM_EXEGESIS_TARGETS is unset so the parent LLVM_EXEGESIS_TARGETS
is set to “ X86”, but in the second the local LLVM_EXEGESIS_TARGETS is still unset so the parent
LLVM_EXEGESIS_TARGETS gets set to “ AArch64”.

As for the correct way to fix it, looking a bit at the .cmake files in llvm/cmake/modules/ it looks like the standard
way would be to have the file set LLVM_EXEGESIS_TARGETS locally, then in the parent. i.e. something like

if (LLVM_TARGETS_TO_BUILD MATCHES "X86")
  set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} X86")
endif()
if (LLVM_TARGETS_TO_BUILD MATCHES "AArch64")
  set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} AArch64")
endif()
set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS}" PARENT_SCOPE)

John

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Steven Noonan via llvm-dev
Sent: 20 September 2018 16:42
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] llvm-exegesis broken on x86

Something's up with the llvm-exegesis build process on 7.0.0 release. On my x86 machines they always complain "no exegesis target for x86_64-pc-linux-gnu, using default", and this seems to be because LLVM_EXEGESIS_INITIALIZE_NATIVE_TARGET never gets set by CMake.

The CMake scripts try to set LLVM_EXEGESIS_TARGETS in the lib subdirectory, but this doesn't seem to work properly. I made this change to figure out what's happening:

---

diff --git a/tools/llvm-exegesis/CMakeLists.txt b/tools/llvm-exegesis/CMakeLists.txt
index 65b1ada8529..94842519ca7 100644
--- a/tools/llvm-exegesis/CMakeLists.txt
+++ b/tools/llvm-exegesis/CMakeLists.txt
@@ -8,7 +8,9 @@ add_llvm_tool(llvm-exegesis
   llvm-exegesis.cpp
   )

+message(WARNING "LLVM_EXEGESIS_TARGETS at parent scope before: ${LLVM_EXEGESIS_TARGETS}")
 add_subdirectory(lib)
+message(FATAL_ERROR "LLVM_EXEGESIS_TARGETS at parent scope after: ${LLVM_EXEGESIS_TARGETS}")

 # Link the native exegesis target if compiled and on the right host.
 if ((LLVM_TARGETS_TO_BUILD MATCHES "${LLVM_NATIVE_ARCH}") AND (LLVM_EXEGESIS_TARGETS MATCHES "${LLVM_NATIVE_ARCH}"))
diff --git a/tools/llvm-exegesis/lib/CMakeLists.txt b/tools/llvm-exegesis/lib/CMakeLists.txt
index 175c2adf9de..4dc91ef125c 100644
--- a/tools/llvm-exegesis/lib/CMakeLists.txt
+++ b/tools/llvm-exegesis/lib/CMakeLists.txt
@@ -1,12 +1,18 @@
+message( WARNING "LLVM_EXEGESIS_TARGETS before: ${LLVM_EXEGESIS_TARGETS}" )
+
 if (LLVM_TARGETS_TO_BUILD MATCHES "X86")
   add_subdirectory(X86)
+  message( WARNING "Appending X86" )
   set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} X86" PARENT_SCOPE)
 endif()
 if (LLVM_TARGETS_TO_BUILD MATCHES "AArch64")
   add_subdirectory(AArch64)
+  message( WARNING "Appending AArch64" )
   set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} AArch64" PARENT_SCOPE)
 endif()

+message( WARNING "LLVM_EXEGESIS_TARGETS after: ${LLVM_EXEGESIS_TARGETS}" )
+
 add_library(LLVMExegesis
   STATIC
   Analysis.cpp

---

This results in this behavior (extraneous newlines removed for readability):

CMake Warning at tools/llvm-exegesis/CMakeLists.txt:11 (message):
  LLVM_EXEGESIS_TARGETS at parent scope before:
CMake Warning at tools/llvm-exegesis/lib/CMakeLists.txt:1 (message):
  LLVM_EXEGESIS_TARGETS before:
CMake Warning at tools/llvm-exegesis/lib/CMakeLists.txt:5 (message):
  Appending X86
CMake Warning at tools/llvm-exegesis/lib/CMakeLists.txt:10 (message):
  Appending AArch64
CMake Warning at tools/llvm-exegesis/lib/CMakeLists.txt:14 (message):
  LLVM_EXEGESIS_TARGETS after:
CMake Error at tools/llvm-exegesis/CMakeLists.txt:13 (message):
  LLVM_EXEGESIS_TARGETS at parent scope after: AArch64

So reading the LLVM_EXEGESIS_TARGETS variable in lib/CMakeLists.txt doesn't work properly, and the second set(... PARENT_SCOPE) just sees the empty LLVM_EXEGESIS_TARGETS value, overwriting the previously appended X86 value.

Any ideas on how to fix this? I'm doing this locally but it seems sloppy:

diff --git a/tools/llvm-exegesis/lib/CMakeLists.txt b/tools/llvm-exegesis/lib/CMakeLists.txt
index 175c2adf9de..194304adf98 100644
--- a/tools/llvm-exegesis/lib/CMakeLists.txt
+++ b/tools/llvm-exegesis/lib/CMakeLists.txt
@@ -1,12 +1,16 @@
+set(TARGETS_TO_APPEND "")
+
 if (LLVM_TARGETS_TO_BUILD MATCHES "X86")
   add_subdirectory(X86)
-  set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} X86" PARENT_SCOPE)
+  set(TARGETS_TO_APPEND "${TARGETS_TO_APPEND} X86")
 endif()
 if (LLVM_TARGETS_TO_BUILD MATCHES "AArch64")
   add_subdirectory(AArch64)
-  set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} AArch64" PARENT_SCOPE)
+  set(TARGETS_TO_APPEND "${TARGETS_TO_APPEND} AArch64")
 endif()

+set(LLVM_EXEGESIS_TARGETS "${LLVM_EXEGESIS_TARGETS} ${TARGETS_TO_APPEND}" PARENT_SCOPE)
+
 add_library(LLVMExegesis
   STATIC
   Analysis.cpp
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