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| <title>Using as: ARM Unwinding Tutorial</title> |
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| <a name="ARM-Unwinding-Tutorial"></a> |
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| <p> |
| Previous: <a href="ARM-Mapping-Symbols.html#ARM-Mapping-Symbols" accesskey="p" rel="prev">ARM Mapping Symbols</a>, Up: <a href="ARM_002dDependent.html#ARM_002dDependent" accesskey="u" rel="up">ARM-Dependent</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="AS-Index.html#AS-Index" title="Index" rel="index">Index</a>]</p> |
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| <hr> |
| <a name="Unwinding"></a> |
| <h4 class="subsection">9.4.7 Unwinding</h4> |
| |
| <p>The ABI for the ARM Architecture specifies a standard format for |
| exception unwind information. This information is used when an |
| exception is thrown to determine where control should be transferred. |
| In particular, the unwind information is used to determine which |
| function called the function that threw the exception, and which |
| function called that one, and so forth. This information is also used |
| to restore the values of callee-saved registers in the function |
| catching the exception. |
| </p> |
| <p>If you are writing functions in assembly code, and those functions |
| call other functions that throw exceptions, you must use assembly |
| pseudo ops to ensure that appropriate exception unwind information is |
| generated. Otherwise, if one of the functions called by your assembly |
| code throws an exception, the run-time library will be unable to |
| unwind the stack through your assembly code and your program will not |
| behave correctly. |
| </p> |
| <p>To illustrate the use of these pseudo ops, we will examine the code |
| that G++ generates for the following C++ input: |
| </p> |
| <pre class="verbatim">void callee (int *); |
| |
| int |
| caller () |
| { |
| int i; |
| callee (&i); |
| return i; |
| } |
| </pre> |
| <p>This example does not show how to throw or catch an exception from |
| assembly code. That is a much more complex operation and should |
| always be done in a high-level language, such as C++, that directly |
| supports exceptions. |
| </p> |
| <p>The code generated by one particular version of G++ when compiling the |
| example above is: |
| </p> |
| <pre class="verbatim">_Z6callerv: |
| .fnstart |
| .LFB2: |
| @ Function supports interworking. |
| @ args = 0, pretend = 0, frame = 8 |
| @ frame_needed = 1, uses_anonymous_args = 0 |
| stmfd sp!, {fp, lr} |
| .save {fp, lr} |
| .LCFI0: |
| .setfp fp, sp, #4 |
| add fp, sp, #4 |
| .LCFI1: |
| .pad #8 |
| sub sp, sp, #8 |
| .LCFI2: |
| sub r3, fp, #8 |
| mov r0, r3 |
| bl _Z6calleePi |
| ldr r3, [fp, #-8] |
| mov r0, r3 |
| sub sp, fp, #4 |
| ldmfd sp!, {fp, lr} |
| bx lr |
| .LFE2: |
| .fnend |
| </pre> |
| <p>Of course, the sequence of instructions varies based on the options |
| you pass to GCC and on the version of GCC in use. The exact |
| instructions are not important since we are focusing on the pseudo ops |
| that are used to generate unwind information. |
| </p> |
| <p>An important assumption made by the unwinder is that the stack frame |
| does not change during the body of the function. In particular, since |
| we assume that the assembly code does not itself throw an exception, |
| the only point where an exception can be thrown is from a call, such |
| as the <code>bl</code> instruction above. At each call site, the same saved |
| registers (including <code>lr</code>, which indicates the return address) |
| must be located in the same locations relative to the frame pointer. |
| </p> |
| <p>The <code>.fnstart</code> (see <a href="ARM-Directives.html#arm_005ffnstart">.fnstart pseudo op</a>) pseudo |
| op appears immediately before the first instruction of the function |
| while the <code>.fnend</code> (see <a href="ARM-Directives.html#arm_005ffnend">.fnend pseudo op</a>) pseudo |
| op appears immediately after the last instruction of the function. |
| These pseudo ops specify the range of the function. |
| </p> |
| <p>Only the order of the other pseudos ops (e.g., <code>.setfp</code> or |
| <code>.pad</code>) matters; their exact locations are irrelevant. In the |
| example above, the compiler emits the pseudo ops with particular |
| instructions. That makes it easier to understand the code, but it is |
| not required for correctness. It would work just as well to emit all |
| of the pseudo ops other than <code>.fnend</code> in the same order, but |
| immediately after <code>.fnstart</code>. |
| </p> |
| <p>The <code>.save</code> (see <a href="ARM-Directives.html#arm_005fsave">.save pseudo op</a>) pseudo op |
| indicates registers that have been saved to the stack so that they can |
| be restored before the function returns. The argument to the |
| <code>.save</code> pseudo op is a list of registers to save. If a register |
| is “callee-saved” (as specified by the ABI) and is modified by the |
| function you are writing, then your code must save the value before it |
| is modified and restore the original value before the function |
| returns. If an exception is thrown, the run-time library restores the |
| values of these registers from their locations on the stack before |
| returning control to the exception handler. (Of course, if an |
| exception is not thrown, the function that contains the <code>.save</code> |
| pseudo op restores these registers in the function epilogue, as is |
| done with the <code>ldmfd</code> instruction above.) |
| </p> |
| <p>You do not have to save callee-saved registers at the very beginning |
| of the function and you do not need to use the <code>.save</code> pseudo op |
| immediately following the point at which the registers are saved. |
| However, if you modify a callee-saved register, you must save it on |
| the stack before modifying it and before calling any functions which |
| might throw an exception. And, you must use the <code>.save</code> pseudo |
| op to indicate that you have done so. |
| </p> |
| <p>The <code>.pad</code> (see <a href="ARM-Directives.html#arm_005fpad">.pad</a>) pseudo op indicates a |
| modification of the stack pointer that does not save any registers. |
| The argument is the number of bytes (in decimal) that are subtracted |
| from the stack pointer. (On ARM CPUs, the stack grows downwards, so |
| subtracting from the stack pointer increases the size of the stack.) |
| </p> |
| <p>The <code>.setfp</code> (see <a href="ARM-Directives.html#arm_005fsetfp">.setfp pseudo op</a>) pseudo op |
| indicates the register that contains the frame pointer. The first |
| argument is the register that is set, which is typically <code>fp</code>. |
| The second argument indicates the register from which the frame |
| pointer takes its value. The third argument, if present, is the value |
| (in decimal) added to the register specified by the second argument to |
| compute the value of the frame pointer. You should not modify the |
| frame pointer in the body of the function. |
| </p> |
| <p>If you do not use a frame pointer, then you should not use the |
| <code>.setfp</code> pseudo op. If you do not use a frame pointer, then you |
| should avoid modifying the stack pointer outside of the function |
| prologue. Otherwise, the run-time library will be unable to find |
| saved registers when it is unwinding the stack. |
| </p> |
| <p>The pseudo ops described above are sufficient for writing assembly |
| code that calls functions which may throw exceptions. If you need to |
| know more about the object-file format used to represent unwind |
| information, you may consult the <cite>Exception Handling ABI for the |
| ARM Architecture</cite> available from <a href="http://infocenter.arm.com">http://infocenter.arm.com</a>. |
| </p> |
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