swr: Refactor checks for compiler feature flags

Encapsulate the test for which flags are needed to get a compiler to
support certain features.  Along with this, give various options to try
for AVX and AVX2 support.  Ideally we want to use specific instruction
set feature flags, like -mavx2 for instance instead of -march=haswell,
but the flags required for certain compilers are different.  This
allows, for AVX2 for instance, GCC to use -mavx2 -mfma -mbmi2 -mf16c
while the Intel compiler which doesn't support those flags can fall
back to using -march=core-avx2.

This addresses a bug where the Intel compiler will silently ignore the
AVX2 instruction feature flags and then potentially fail to build.

v2: Pass preprocessor-check argument as true-state instead of
    false-state for clarity.
v3: Reduce AVX2 define test to just __AVX2__.  Additional defines suchas
    __FMA__, __BMI2__, and __F16C__ appear to be inconsistently defined
    w.r.t thier availability.
v4: Fix C++11 flags being added globally and add more logic to
    swr_require_cxx_feature_flags

Cc: <[email protected]>
Reviewed-by: Tim Rowley <[email protected]>
Tested-by: Tim Rowley <[email protected]>
Signed-off-by: Chuck Atkins <[email protected]>

1 files changed, 50 insertions, 23 deletions

diff --git a/configure.ac b/configure.ac
index cc9bc4758b8..8321e8eb21d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -2330,6 +2330,45 @@ swr_llvm_check() {
     fi
 }
 
+swr_require_cxx_feature_flags() {
+    feature_name="$1"
+    preprocessor_test="$2"
+    option_list="$3"
+    output_var="$4"
+
+    AC_MSG_CHECKING([whether $CXX supports $feature_name])
+    AC_LANG_PUSH([C++])
+    save_CXXFLAGS="$CXXFLAGS"
+    save_IFS="$IFS"
+    IFS=","
+    found=0
+    for opts in $option_list
+    do
+        unset IFS
+        CXXFLAGS="$opts $save_CXXFLAGS"
+        AC_COMPILE_IFELSE(
+            [AC_LANG_PROGRAM(
+                [   #if !($preprocessor_test)
+                    #error
+                    #endif
+                ])],
+            [found=1; break],
+            [])
+        IFS=","
+    done
+    IFS="$save_IFS"
+    CXXFLAGS="$save_CXXFLAGS"
+    AC_LANG_POP([C++])
+    if test $found -eq 1; then
+        AC_MSG_RESULT([$opts])
+        eval "$output_var=\$opts"
+        return 0
+    fi
+    AC_MSG_RESULT([no])
+    AC_MSG_ERROR([swr requires $feature_name support])
+    return 1
+}
+
 dnl Duplicates in GALLIUM_DRIVERS_DIRS are removed by sorting it after this block
 if test -n "$with_gallium_drivers"; then
     gallium_drivers=`IFS=', '; echo $with_gallium_drivers`
@@ -2399,31 +2438,19 @@ if test -n "$with_gallium_drivers"; then
         xswr)
             swr_llvm_check "swr"
 
-            AC_MSG_CHECKING([whether $CXX supports c++11/AVX/AVX2])
-            SWR_AVX_CXXFLAGS="-mavx"
-            SWR_AVX2_CXXFLAGS="-mavx2 -mfma -mbmi2 -mf16c"
-
-            AC_LANG_PUSH([C++])
-            save_CXXFLAGS="$CXXFLAGS"
-            CXXFLAGS="-std=c++11 $CXXFLAGS"
-            AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],[],
-                              [AC_MSG_ERROR([c++11 compiler support not detected])])
-            CXXFLAGS="$save_CXXFLAGS"
-
-            save_CXXFLAGS="$CXXFLAGS"
-            CXXFLAGS="$SWR_AVX_CXXFLAGS $CXXFLAGS"
-            AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],[],
-                              [AC_MSG_ERROR([AVX compiler support not detected])])
-            CXXFLAGS="$save_CXXFLAGS"
-
-            save_CFLAGS="$CXXFLAGS"
-            CXXFLAGS="$SWR_AVX2_CXXFLAGS $CXXFLAGS"
-            AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],[],
-                              [AC_MSG_ERROR([AVX2 compiler support not detected])])
-            CXXFLAGS="$save_CXXFLAGS"
-            AC_LANG_POP([C++])
+            swr_require_cxx_feature_flags "C++11" "__cplusplus >= 201103L" \
+                ",-std=c++11" \
+                SWR_CXX11_CXXFLAGS
+            AC_SUBST([SWR_CXX11_CXXFLAGS])
 
+            swr_require_cxx_feature_flags "AVX" "defined(__AVX__)" \
+                ",-mavx,-march=core-avx" \
+                SWR_AVX_CXXFLAGS
             AC_SUBST([SWR_AVX_CXXFLAGS])
+
+            swr_require_cxx_feature_flags "AVX2" "defined(__AVX2__)" \
+                ",-mavx2 -mfma -mbmi2 -mf16c,-march=core-avx2" \
+                SWR_AVX2_CXXFLAGS
             AC_SUBST([SWR_AVX2_CXXFLAGS])
 
             HAVE_GALLIUM_SWR=yes