add support for __switch* needed for switch statements in thumb codegen

llvm-svn: 82184

compiler-rt/lib/arm/switch.S

@@ -0,0 +1,93 @@
+//===-- switch.S - Implement switch* --------------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+//
+// When compiling switch statements in thumb mode, the compiler 
+// can use these __switch* helper functions  The compiler emits a blx to 
+// the __switch* function followed by a table of displacements for each 
+// case statement.  On entry, R0 is the index into the table. The __switch* 
+// function uses the return address in lr to find the start of the table.
+// The first entry in the table is the count of the entries in the table.
+// It then uses R0 to index into the table and get the displacement of the 
+// address to jump to.  If R0 is greater than the size of the table, it jumps
+// to the last entry in the table. Each displacement in the table is actually 
+// the distance from lr to the label, thus making the tables PIC.
+
+
+	.text
+	.syntax unified 
+
+//
+// The table contains unsigned byte sized elements which are 1/2 the distance
+// from lr to the target label.
+//
+	.globl ___switchu8
+	.private_extern ___switchu8
+___switchu8:
+	ldrb    ip, [lr, #-1]           // get first byte in table
+	cmp     r0, ip                  // compare with index
+	ldrbcc  r0, [lr, r0]            // get indexed byte out of table
+	ldrbhs  r0, [lr, ip]            // if out of range, use last entry in table
+	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
+	bx      ip                      // jump to computed label
+
+
+
+//
+// The table contains signed byte sized elements which are 1/2 the distance
+// from lr to the target label.  
+//
+	.globl ___switch8
+	.private_extern ___switch8
+___switch8:
+	ldrb    ip, [lr, #-1]           // get first byte in table
+	cmp     r0, ip                  // signed compare with index
+	ldrsbcc r0, [lr, r0]            // get indexed byte out of table
+	ldrsbhs r0, [lr, ip]            // if out of range, use last entry in table
+	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
+	bx      ip                      // jump to computed label
+
+
+//
+// The table contains signed 2-byte sized elements which are 1/2 the distance
+// from lr to the target label.    
+//
+	.globl ___switch16
+	.private_extern ___switch16
+___switch16:
+	ldrh    ip, [lr, #-1]           // get first 16-bit word in table
+	cmp     r0, ip                  // compare with index
+	add     r0, lr, r0, lsl #1      // compute address of element in table
+	ldrshcc r0, [r0, #1]            // load 16-bit element if r0 is in range
+	add     ip, lr, ip, lsl #1      // compute address of last element in table
+	ldrshhs r0, [ip, #1]            // load 16-bit element if r0 out of range
+	add     ip, lr, r0, lsl #1      // compute label = lr + element*2
+	bx      ip                      // jump to computed label
+
+
+//
+// The table contains signed 4-byte sized elements which are the distance
+// from lr to the target label.    
+//
+	.globl ___switch32
+	.private_extern ___switch32
+___switch32:
+	ldr     ip, [lr, #-1]            // get first 32-bit word in table
+	cmp     r0, ip                   // compare with index
+	add     r0, lr, r0, lsl #2       // compute address of element in table
+	ldrcc   r0, [r0, #3]             // load 32-bit element if r0 is in range
+	add     ip, lr, ip, lsl #2       // compute address of last element in table
+	ldrcs   r0, [ip, #3]             // load 32-bit element if r0 out of range
+	add     ip, lr, r0               // compute label = lr + element
+	bx      ip                       // jump to computed label
+
+
+	// tell linker it can break up file at label boundaries
+	.subsections_via_symbols
+