picocrt/lm32: Split out _start to an asm file

This avoids problems if the compiler decides to use the stack before
we've gotten it initialized, as happens when building without
optimization.

Signed-off-by: Keith Packard <[email protected]>

Author: keith-packard

picocrt/machine/lm32/crt0.S

@@ -0,0 +1,52 @@
+/*
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ * Copyright © 2025 Keith Packard
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above
+ *    copyright notice, this list of conditions and the following
+ *    disclaimer in the documentation and/or other materials provided
+ *    with the distribution.
+ *
+ * 3. Neither the name of the copyright holder nor the names of its
+ *    contributors may be used to endorse or promote products derived
+ *    from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT,
+ * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
+ * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+ * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
+ * OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+	// Defined in crt0.c
+	.global _cstart
+	.type cstart, %function
+
+	// _start: Main entry point function, sets up the hardware to the point where
+	// we can execute C code. 
+	.section .text.init.enter, "ax", %progbits
+	.global _start
+	.type _start, %function
+_start:
+	# Initialize r0
+	xor	r0, r0, r0
+	mvhi	sp, hi(__stack-8)
+	ori 	sp, sp, lo(__stack-8)
+
+	/* Branch to C code */
+        bi	_cstart

picocrt/machine/lm32/crt0.c

@@ -35,21 +35,8 @@
 
 #include "../../crt0.h"
 
-static void __attribute__((used)) __section(".init")
+void __section(".init")
 _cstart(void)
 {
 	__start();
 }
-
-extern char __stack[];
-
-void __section(".init") __attribute__((used))
-_start(void)
-{
-	/* Initialize stack pointer */
-	__asm__("mvhi sp, hi(%0)" : : "i" (__stack));
-	__asm__("ori  sp, sp, lo(%0)" : : "i" (__stack));
-
-	/* Branch to C code */
-        __asm__("bi %0" : : "i" (_cstart));
-}

picocrt/machine/lm32/meson.build

@@ -32,4 +32,4 @@
 # ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 # OF THE POSSIBILITY OF SUCH DAMAGE.
 #
-src_picocrt += files('crt0.c')
+src_picocrt += files('crt0.c', 'crt0.S')