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[avr-gcc-list] Frame pointer location

From: Johannes Bauer
Subject: [avr-gcc-list] Frame pointer location
Date: Fri, 13 Aug 2010 10:32:42 +0200
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Hello list,

I've got another question regarding the AVR ISR prologues: Let's say I compile with -fno-omit-frame-pointer. Then the Y-ptr (r28:29) is used as a FP. The resulting code for functions looks like this:

000087b4 <testfnc>:
    87b4:       af 92           push    r10
    87b6:       bf 92           push    r11
    87b8:       cf 92           push    r12
    87ba:       df 92           push    r13
    87bc:       ef 92           push    r14
    87be:       ff 92           push    r15
    87c0:       0f 93           push    r16
    87c2:       1f 93           push    r17
    87c4:       df 93           push    r29
    87c6:       cf 93           push    r28
    87c8:       cd b7           in      r28, 0x3d       ; 61
    87ca:       de b7           in      r29, 0x3e       ; 62

or, for an ISR

00008962 <__vector_96>:
    8962:       1f 92           push    r1
    8964:       0f 92           push    r0
    8966:       0f b6           in      r0, 0x3f        ; 63
    8968:       0f 92           push    r0
    896a:       08 b6           in      r0, 0x38        ; 56
    896c:       0f 92           push    r0
    896e:       18 be           out     0x38, r1        ; 56
    8970:       09 b6           in      r0, 0x39        ; 57
    8972:       0f 92           push    r0
    8974:       19 be           out     0x39, r1        ; 57
    8976:       0b b6           in      r0, 0x3b        ; 59
    8978:       0f 92           push    r0
    897a:       1b be           out     0x3b, r1        ; 59
    897c:       11 24           eor     r1, r1
    897e:       0f 93           push    r16
    8980:       1f 93           push    r17
    8982:       2f 93           push    r18
    8984:       3f 93           push    r19
    8986:       4f 93           push    r20
    8988:       5f 93           push    r21
    898a:       6f 93           push    r22
    898c:       7f 93           push    r23
    898e:       8f 93           push    r24
    8990:       9f 93           push    r25
    8992:       af 93           push    r26
    8994:       bf 93           push    r27
    8996:       ef 93           push    r30
    8998:       ff 93           push    r31
    899a:       df 93           push    r29
    899c:       cf 93           push    r28
    899e:       cd b7           in      r28, 0x3d       ; 61
    89a0:       de b7           in      r29, 0x3e       ; 62

i.e. the frame pointer is the *last* thing which changes during the prologue. This makes sense since the amount of local variables reachable via ldd/std "Y+n" operations is greater than when the frame pointer would have been pushed as the first operation.

It has one major drawback, though: When trying to unwind the stack manually, i.e., when I try to create a stacktrace, this is often not possible, since I cannot rely on the return address being at Y+3/+4/+5. The stack unwinding works, however, since Y+1/+2 always holds the last frame pointer.

Is there an option to *always* make the frame pointer get pushed out first? In an ISR that would have to happen even before SREG is pushed onto the stack. It would make stacktraces possible.

If it is not possible and I would like to change that myself for debugging - what parts would I have to look at? Is it easily done or is that a difficult change?


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