Revert "Auto-detect PEXT"

Also provide a CC hook for compiler.

official

@@ -25,7 +25,7 @@ popcnt:
 	$(CC) -mpopcnt $(CF) -DVERSION=\"$(VERSION)\" ./src/*.c -o $(EXE) -static $(LF)
 
 pext:
-	$(CC) -mpopcnt -mbmi2 $(CF) -DVERSION=\"$(VERSION)\" ./src/*.c -o $(EXE) -static $(LF)
+	$(CC) -mpopcnt -mbmi2 -DPEXT $(CF) -DVERSION=\"$(VERSION)\" ./src/*.c -o $(EXE) -static $(LF)
 
 clean:
 	rm $(EXE)

src/bitboard.c

@@ -16,7 +16,7 @@
 #include <stdio.h>
 #include "bitboard.h"
 
-#ifdef __BMI2__
+#ifdef PEXT
     #include <immintrin.h> // Header for _pext_u64() intrinsic
 #endif
 
@@ -101,7 +101,7 @@ static bitboard_t slider_attacks(int square, bitboard_t occ, const int dir[4][2]
 
 static int slider_index(bitboard_t occ, bitboard_t mask, bitboard_t magic, unsigned shift)
 {
-#ifdef __BMI2__
+#ifdef PEXT
     (void)magic, (void)shift;  // Silence compiler warnings (unused variables)
     return _pext_u64(occ, mask);
 #else

src/makefile

@@ -1,8 +1,7 @@
 # makefile for dev and OpenBench
 
-# TODO:
-# * auto-detect compiler: clang if available (8% faster), otherwise gcc. For now assume gcc only
-#   for openbench.
+# Compiler: gcc by default (for OpenBench), but clang produces faster code
+CC = gcc
 
 # Compilation flags
 CF = -std=gnu11 -DNDEBUG -O3 -flto -Wfatal-errors -Wall -Wextra -Wshadow
@@ -13,14 +12,13 @@ LF = -s -lm -lpthread
 # EXE is the hook used by OpenBench to specify the output file name
 EXE = demolito
 
-# POPCNT: auto-detection (for both targets), thanks to march=native and compiler intrinsic.
-# PEXT: off for openbench target (we can't assume that all OpenBench workers have it).
+# pext is only for Intel CPU with BMI2 support (haswell+). Do not use it for AMD.
 
-openbench:
-	gcc -march=native $(CF) -DVERSION=\"dev\" ./*.c -o $(EXE) $(LF)
+default:
+	$(CC) -march=native $(CF) -DVERSION=\"dev\" ./*.c -o $(EXE) $(LF)
 
-dev:
-	clang -march=native $(CF) -DVERSION=\"dev\" ./*.c -o $(EXE) $(LF)
+pext:
+	$(CC) -march=native -DPEXT $(CF) -DVERSION=\"dev\" ./*.c -o $(EXE) $(LF)
 
 clean:
 	rm $(EXE)