add assembly implementation of modsi3 so compiler does not have to special case a - (a / b) * b optimization

git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@109492 91177308-0d34-0410-b5e6-96231b3b80d8

Author: Nick Kledzik

lib/arm/modsi3.S

@@ -0,0 +1,36 @@
+//===-------- modsi3.S - Implement modsi3 ---------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "../assembly.h"
+
+//
+// extern int32_t __modsi3(int32_t a, int32_t b);
+//
+// Returns the remainder when dividing two 32-bit signed integers.
+// Conceptually, the function is: { return a - (a / b) * b; }
+// But if you write that in C, llvm compiles it to a call to __modsi3...
+//
+	.align 2
+DEFINE_COMPILERRT_FUNCTION(__modsi3)
+	push	{r4, r5, r7, lr}
+	add	r7, sp, #8	// set stack frame
+	mov	r5, r0		// save a
+	mov	r4, r1		// save b
+	bl      ___divsi3	// compute a/b
+#if __ARM_ARCH_7A__
+	mls     r0, r4, r0, r5  // mulitple result * b and subtract from a
+#else	
+	// before armv7, does not have "mls" instruction
+	mul	r3, r0, r4	// multiple result * b
+	sub     r0, r5, r3      // a - result
+#endif
+	pop     {r4, r5, r7, pc}
+	
+
+