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authorMarçal Comajoan Cara <mcomajoancara@gmail.com>2018-11-18 11:11:04 +0100
committerJoel Sherrill <joel@rtems.org>2018-11-19 13:11:55 -0600
commitae05a2783765b26f66d087bc8325f4a241260fd4 (patch)
treed5b8f37f537545e472dbb983e789057535f2a6ce /cpu-supplement/sparc_v8_stacks_regwin.rst
parent0bb0b8d570c6bb866f158e3a91f94b0463ae290e (diff)
downloadrtems-docs-ae05a2783765b26f66d087bc8325f4a241260fd4.tar.bz2
Improve SPARC Calling Overview Webpage conversion
Fixed tables, typos, redrawn images and converted ASCII art to ditaa and PNG, and improved the overall format. This work was part of GCI 2018. Closes #3567.
Diffstat (limited to 'cpu-supplement/sparc_v8_stacks_regwin.rst')
-rw-r--r--cpu-supplement/sparc_v8_stacks_regwin.rst284
1 files changed, 75 insertions, 209 deletions
diff --git a/cpu-supplement/sparc_v8_stacks_regwin.rst b/cpu-supplement/sparc_v8_stacks_regwin.rst
index 155e905..37df078 100644
--- a/cpu-supplement/sparc_v8_stacks_regwin.rst
+++ b/cpu-supplement/sparc_v8_stacks_regwin.rst
@@ -5,12 +5,6 @@
.. comment This content is no longer online and only accessible at
.. comment https://web.archive.org/web/20120205014832/https://www.sics.se/~psm/sparcstack.html
-.. comment XXX Format Tables
-.. comment XXX Format Figures (could be code, ascii art, etc.)
-.. comment XXX double check against web page
-.. comment XXX Fix Figure references in text
-.. comment XXX instruction names probably should be marked as code font
-
Understanding stacks and registers in the SPARC architecture(s)
===============================================================
The SPARC architecture from Sun Microsystems has some "interesting"
@@ -23,11 +17,11 @@ me know. This document is limited to the V8 version of the architecture.
General Structure
-----------------
SPARC has 32 general purpose integer registers visible to the program
-at any given time. Of these, 8 registers are global registers and 24
+at any given time. Of these, 8 registers are ``global`` registers and 24
registers are in a register window. A window consists of three groups
-of 8 registers, the out, local, and in registers. See table 1. A SPARC
-implementation can have from 2 to 32 windows, thus varying the number
-of registers from 40 to 520. Most implentations have 7 or 8 windows. The
+of 8 registers, the ``out``, ``local``, and ``in`` registers. See table 1. A
+SPARC implementation can have from 2 to 32 windows, thus varying the number
+of registers from 40 to 520. Most implementations have 7 or 8 windows. The
variable number of registers is the principal reason for the SPARC being
"scalable".
@@ -36,12 +30,12 @@ current window pointer (CWP) which is part of the processor status
register (PSR). This is a five bit value that can be decremented or
incremented by the SAVE and RESTORE instructions, respectively. These
instructions are generally executed on procedure call and return
-(respectively). The idea is that the in registers contain incoming
-parameters, the local register constitute scratch registers, the out
-registers contain outgoing parameters, and the global registers contain
+(respectively). The idea is that the ``in`` registers contain incoming
+parameters, the ``local`` register constitutes scratch registers, the ``out``
+registers contain outgoing parameters, and the ``global`` registers contain
values that vary little between executions. The register windows overlap
-partially, thus the out registers become renamed by SAVE to become the in
-registers of the called procedure. Thus, the memory traffic is reduced
+partially, thus the ``out`` registers become renamed by SAVE to become the
+``in`` registers of the called procedure. Thus, the memory traffic is reduced
when going up and down the procedure call. Since this is a frequent
operation, performance is improved.
@@ -54,39 +48,38 @@ opposed to multitasking workloads, and by considering compilers with
poor optimization. It also caused considerable problems in implementing
high-end SPARC processors such as the SuperSPARC, although more recent
implementations have dealt effectively with the obstacles. Register
-windows is now part of the compatibility legacy and not easily removed
+windows are now part of the compatibility legacy and not easily removed
from the architecture.)
-.. comment XXX FIX FORMATTING
+.. table:: Table 1 - Visible Registers
+
+ +----------------+------------+---------------+
+ | Register | Mnemonic | Register |
+ | Group | | Address |
+ +================+============+===============+
+ + ``global`` + %g0-%g7 + r[0] - r[7] +
+ +----------------+------------+---------------+
+ + ``out`` + %o0-%o7 + r[8] - r[15] +
+ +----------------+------------+---------------+
+ + ``local`` + %l0-%l7 + r[16] - r[23] +
+ +----------------+------------+---------------+
+ + ``in`` + %i0-%i7 + r[24] - r[31] +
+ +----------------+------------+---------------+
-+------------+------------+---------------+
-| Register | Mnemonic | Register |
-| Group | | Address |
-+============+============+===============+
-+ global + %g0-%g7 + r[0] - r[7] +
-+------------+------------+---------------+
-+ out + %o0-%o7 + r[8] - r[15] +
-+------------+------------+---------------+
-+ local + %l0-%l7 + r[16] - r[23] +
-+------------+------------+---------------+
-+ in + %i0-%i7 + r[24] - r[31] +
-+------------+------------+---------------+
-
-Table 1 - Visible Registers
The overlap of the registers is illustrated in figure 1. The figure
shows an implementation with 8 windows, numbered 0 to 7 (labeled w0 to
-w7 in the figure).. Each window corresponds to 24 registers, 16 of which
+w7 in the figure). Each window corresponds to 24 registers, 16 of which
are shared with "neighboring" windows. The windows are arranged in a
wrap-around manner, thus window number 0 borders window number 7. The
common cause of changing the current window, as pointed to by CWP, is
-the RESTORE and SAVE instuctions, shown in the middle. Less common is
+the RESTORE and SAVE instructions, shown in the middle. Less common is
the supervisor RETT instruction (return from trap) and the trap event
(interrupt, exception, or TRAP instruction).
-.. comment XXX insert graphic from website (redraw if needed)
+.. figure:: ../images/cpu_supplement/sparcwin.png
-Figure 1 - Windowed Registers
+ Figure 1 - Windowed Registers
The "WIM" register is also indicated in the top left of Figure 1. The
window invalid mask is a bit map of valid windows. It is generally used
@@ -111,8 +104,6 @@ well (including header files and compiler documentation).
Figure 2 shows a summary of register contents at any given time.
-.. comment XXX FIX FORMATTING
-
.. code-block:: c
%g0 (r00) always zero
@@ -151,13 +142,15 @@ Figure 2 shows a summary of register contents at any given time.
%fp, %i6 (r30) [3] frame pointer
%i7 (r31) [3] return address - 8
-Notes:
+.. topic:: Items
+
+ [1] assumed by caller to be destroyed (volatile) across a procedure call
-[1] assumed by caller to be destroyed (volatile) across a procedure call
-[2] should not be used by SPARC ABI library code
-[3] assumed by caller to be preserved across a procedure call
+ [2] should not be used by SPARC ABI library code
-Figure 2 - SPARC register semantics
+ [3] assumed by caller to be preserved across a procedure call
+
+*Figure 2 - SPARC register semantics*
Particular compilers are likely to vary slightly.
@@ -167,7 +160,7 @@ overwrite these registers unless they've been compiled with suitable
flags. Also, the "reserved" registers are presumed to be allocated
(in the future) bottom-up, i.e. %g7 is currently the "safest" to use.
-Optimizing linkers and interpreters are exmples that use global registers.
+Optimizing linkers and interpreters are examples that use global registers.
Register Windows and the Stack
------------------------------
@@ -175,55 +168,17 @@ Register Windows and the Stack
The SPARC register windows are, naturally, intimately related to the
stack. In particular, the stack pointer (%sp or %o6) must always point
to a free block of 64 bytes. This area is used by the operating system
-(Solaris, SunOS, and Linux at least) to save the current local and in
-registers upon a system interupt, exception, or trap instruction. (Note
-that this can occur at any time.)
+(Solaris, SunOS, and Linux at least) to save the current ``local`` and
+``in`` registers upon a system interrupt, exception, or trap instruction.
+(Note that this can occur at any time.)
Other aspects of register relations with memory are programming
-convention. The typical, and recommended, layout of the stack is shown
+convention. The typical and recommended layout of the stack is shown
in figure 3. The figure shows a stack frame.
-.. comment XXX FIX FORMATTING
-
-.. code-block:: c
+.. figure:: ../images/cpu_supplement/stack_frame_contents.png
- low addresses
-
- +-------------------------+
- %sp --> | 16 words for storing |
- | LOCAL and IN registers |
- +-------------------------+
- | one-word pointer to |
- | aggregate return value |
- +-------------------------+
- | 6 words for callee |
- | to store register |
- | arguments |
- +-------------------------+
- | outgoing parameters |
- | past the 6th, if any |
- +-------------------------+
- | space, if needed, for |
- | compiler temporaries |
- | and saved floating- |
- | point registers |
- +-------------------------+
-
- +-------------------------+
- | space dynamically |
- | allocated via the |
- | alloca() library call |
- +-------------------------+
- | space, if needed, for |
- | automatic arrays, |
- | aggregates, and |
- | addressable scalar |
- | automatics |
- +-------------------------+
- %fp -->
- high addresses
-
-Figure 3 - Stack frame contents
+ Figure 3 - Stack frame contents
Note that the top boxes of figure 3 are addressed via the stack pointer
(%sp), as positive offsets (including zero), and the bottom boxes are
@@ -231,100 +186,20 @@ accessed over the frame pointer using negative offsets (excluding zero),
and that the frame pointer is the old stack pointer. This scheme allows
the separation of information known at compile time (number and size
of local parameters, etc) from run-time information (size of blocks
-allocated by alloca()).
+allocated by ``alloca()``).
"addressable scalar automatics" is a fancy name for local variables.
-The clever nature of the stack and frame pointers are that they are always
+The clever nature of the stack and frame pointers is that they are always
16 registers apart in the register windows. Thus, a SAVE instruction will
make the current stack pointer into the frame pointer and, since the SAVE
instruction also doubles as an ADD, create a new stack pointer. Figure 4
illustrates what the top of a stack might look like during execution. (The
listing is from the "pwin" command in the SimICS simulator.)
-.. comment XXX FIX FORMATTING
-
-.. code-block:: c
+.. figure:: ../images/cpu_supplement/sample_stack_contents.png
- REGISTER WINDOWS
-
- +--+---+----------+
- |g0|r00|0x00000000| global
- |g1|r01|0x00000006| registers
- |g2|r02|0x00091278|
- g0-g7 |g3|r03|0x0008ebd0|
- |g4|r04|0x00000000| (note: 'save' and 'trap' decrements CWP,
- |g5|r05|0x00000000| i.e. moves it up on this diagram. 'restore'
- |g6|r06|0x00000000| and 'rett' increments CWP, i.e. down)
- |g7|r07|0x00000000|
- +--+---+----------+
- CWP (2) |o0|r08|0x00000002|
- |o1|r09|0x00000000| MEMORY
- |o2|r10|0x00000001|
- o0-o7 |o3|r11|0x00000001| stack growth
- |o4|r12|0x000943d0|
- |o5|r13|0x0008b400| ^
- |sp|r14|0xdffff9a0| ----\ /|\
- |o7|r15|0x00062abc| | | addresses
- +--+---+----------+ | +--+----------+ virtual physical
- |l0|r16|0x00087c00| \---> |l0|0x00000000| 0xdffff9a0 0x000039a0 top of frame 0
- |l1|r17|0x00027fd4| |l1|0x00000000| 0xdffff9a4 0x000039a4
- |l2|r18|0x00000000| |l2|0x0009df80| 0xdffff9a8 0x000039a8
- l0-l7 |l3|r19|0x00000000| |l3|0x00097660| 0xdffff9ac 0x000039ac
- |l4|r20|0x00000000| |l4|0x00000014| 0xdffff9b0 0x000039b0
- |l5|r21|0x00097678| |l5|0x00000001| 0xdffff9b4 0x000039b4
- |l6|r22|0x0008b400| |l6|0x00000004| 0xdffff9b8 0x000039b8
- |l7|r23|0x0008b800| |l7|0x0008dd60| 0xdffff9bc 0x000039bc
- +--+--+---+----------+ +--+----------+
- CWP+1 (3) |o0|i0|r24|0x00000002| |i0|0x00091048| 0xdffff9c0 0x000039c0
- |o1|i1|r25|0x00000000| |i1|0x00000011| 0xdffff9c4 0x000039c4
- |o2|i2|r26|0x0008b7c0| |i2|0x00091158| 0xdffff9c8 0x000039c8
- i0-i7 |o3|i3|r27|0x00000019| |i3|0x0008d370| 0xdffff9cc 0x000039cc
- |o4|i4|r28|0x0000006c| |i4|0x0008eac4| 0xdffff9d0 0x000039d0
- |o5|i5|r29|0x00000000| |i5|0x00000000| 0xdffff9d4 0x000039d4
- |o6|fp|r30|0xdffffa00| ----\ |fp|0x00097660| 0xdffff9d8 0x000039d8
- |o7|i7|r31|0x00040468| | |i7|0x00000000| 0xdffff9dc 0x000039dc
- +--+--+---+----------+ | +--+----------+
- | |0x00000001| 0xdffff9e0 0x000039e0 parameters
- | |0x00000002| 0xdffff9e4 0x000039e4
- | |0x00000040| 0xdffff9e8 0x000039e8
- | |0x00097671| 0xdffff9ec 0x000039ec
- | |0xdffffa68| 0xdffff9f0 0x000039f0
- | |0x00024078| 0xdffff9f4 0x000039f4
- | |0x00000004| 0xdffff9f8 0x000039f8
- | |0x0008dd60| 0xdffff9fc 0x000039fc
- +--+------+----------+ | +--+----------+
- |l0| |0x00087c00| \---> |l0|0x00091048| 0xdffffa00 0x00003a00 top of frame 1
- |l1| |0x000c8d48| |l1|0x0000000b| 0xdffffa04 0x00003a04
- |l2| |0x000007ff| |l2|0x00091158| 0xdffffa08 0x00003a08
- |l3| |0x00000400| |l3|0x000c6f10| 0xdffffa0c 0x00003a0c
- |l4| |0x00000000| |l4|0x0008eac4| 0xdffffa10 0x00003a10
- |l5| |0x00088000| |l5|0x00000000| 0xdffffa14 0x00003a14
- |l6| |0x0008d5e0| |l6|0x000c6f10| 0xdffffa18 0x00003a18
- |l7| |0x00088000| |l7|0x0008cd00| 0xdffffa1c 0x00003a1c
- +--+--+---+----------+ +--+----------+
- CWP+2 (4) |i0|o0| |0x00000002| |i0|0x0008cb00| 0xdffffa20 0x00003a20
- |i1|o1| |0x00000011| |i1|0x00000003| 0xdffffa24 0x00003a24
- |i2|o2| |0xffffffff| |i2|0x00000040| 0xdffffa28 0x00003a28
- |i3|o3| |0x00000000| |i3|0x0009766b| 0xdffffa2c 0x00003a2c
- |i4|o4| |0x00000000| |i4|0xdffffa68| 0xdffffa30 0x00003a30
- |i5|o5| |0x00064c00| |i5|0x000253d8| 0xdffffa34 0x00003a34
- |i6|o6| |0xdffffa70| ----\ |i6|0xffffffff| 0xdffffa38 0x00003a38
- |i7|o7| |0x000340e8| | |i7|0x00000000| 0xdffffa3c 0x00003a3c
- +--+--+---+----------+ | +--+----------+
- | |0x00000001| 0xdffffa40 0x00003a40 parameters
- | |0x00000000| 0xdffffa44 0x00003a44
- | |0x00000000| 0xdffffa48 0x00003a48
- | |0x00000000| 0xdffffa4c 0x00003a4c
- | |0x00000000| 0xdffffa50 0x00003a50
- | |0x00000000| 0xdffffa54 0x00003a54
- | |0x00000002| 0xdffffa58 0x00003a58
- | |0x00000002| 0xdffffa5c 0x00003a5c
- | | . |
- | | . | .. etc (another 16 bytes)
- | | . |
-
-Figure 4 - Sample stack contents
+ Figure 4 - Sample stack contents
Note how the stack contents are not necessarily synchronized with the
registers. Various events can cause the register windows to be "flushed"
@@ -333,7 +208,7 @@ update by using ST_FLUSH_WINDOWS trap, which also reduces the number of
valid windows to the minimum of 1.
Writing a library for multithreaded execution is an example that requires
-explicit flushing, as is longjmp().
+explicit flushing, as is ``longjmp()``.
Procedure epilogue and prologue
-------------------------------
@@ -341,8 +216,6 @@ Procedure epilogue and prologue
The stack frame described in the previous section leads to the standard
entry/exit mechanisms listed in figure 5.
-.. comment XXX FIX FORMATTING
-
.. code-block:: c
function:
@@ -354,7 +227,7 @@ entry/exit mechanisms listed in figure 5.
ret ; jmpl %i7+8, %g0
restore ; restore %g0,%g0,%g0
-Figure 5 - Epilogue/prologue in procedures
+*Figure 5 - Epilogue/prologue in procedures*
The SAVE instruction decrements the CWP, as discussed earlier, and also
performs an addition. The constant "C" that is used in the figure to
@@ -365,29 +238,27 @@ for the LOCAL and IN registers, i.e. (hex) 0x40 bytes.
A confusing element of the SAVE instruction is that the source operands
(the first two parameters) are read from the old register window, and
the destination operand (the rightmost parameter) is written to the new
-window. Thus, allthough "%sp" is indicated as both source and destination,
+window. Thus, although "%sp" is indicated as both source and destination,
the result is actually written into the stack pointer of the new window
(the source stack pointer becomes renamed and is now the frame pointer).
-The return instructions are also a bit particular. ret is a synthetic
-instruction, corresponding to jmpl (jump linked). This instruction
+The return instructions are also a bit particular. ``ret`` is a synthetic
+instruction, corresponding to ``jmpl`` (jump linked). This instruction
jumps to the address resulting from adding 8 to the %i7 register. The
-source instruction address (the address of the ret instruction itself)
+source instruction address (the address of the ``ret`` instruction itself)
is written to the %g0 register, i.e. it is discarded.
-The restore instruction is similarly a synthetic instruction, and is
-just a short form for a restore that choses not to perform an addition.
+The ``restore`` instruction is similarly a synthetic instruction and is
+just a short form for a restore that chooses not to perform an addition.
The calling instruction, in turn, typically looks as follows:
-.. comment XXX FIX FORMATTING
-
.. code-block:: c
call <function> ; jmpl <address>, %o7
mov 0, %o0
-Again, the call instruction is synthetic, and is actually the same
+Again, the ``call`` instruction is synthetic, and is actually the same
instruction that performs the return. This time, however, it is interested
in saving the return address, into register %o7. Note that the delay
slot is often filled with an instruction related to the parameters,
@@ -397,10 +268,10 @@ Note also that the return value is also generally passed in %o0.
Leaf procedures are different. A leaf procedure is an optimization that
reduces unnecessary work by taking advantage of the knowledge that no
-call instructions exist in many procedures. Thus, the save/restore couple
-can be eliminated. The downside is that such a procedure may only use
-the out registers (since the in and local registers actually belong to
-the caller). See Figure 6.
+``call`` instructions exist in many procedures. Thus, the
+``save``/``restore`` couple can be eliminated. The downside is that such a
+procedure may only use the ``out`` registers (since the ``in`` and ``local``
+registers actually belong to the caller). See Figure 6.
.. comment XXX FIX FORMATTING
@@ -415,11 +286,11 @@ the caller). See Figure 6.
retl ; jmpl %o7+8, %g0
nop ; the delay slot can be used for something else
-Figure 6 - Epilogue/prologue in leaf procedures
+*Figure 6 - Epilogue/prologue in leaf procedures*
Note in the figure that there is only one instruction overhead, namely the
-retl instruction. retl is also synthetic (return from leaf subroutine), is
-again a variant of the jmpl instruction, this time with %o7+8 as target.
+``retl`` instruction. ``retl`` is also synthetic (return from leaf subroutine),
+is again a variant of the ``jmpl`` instruction, this time with %o7+8 as target.
Yet another variation of epilogue is caused by tail call elimination,
an optimization supported by some compilers (including Sun's C compiler
@@ -427,8 +298,6 @@ but not GCC). If the compiler detects that a called function will return
to the calling function, it can replace its place on the stack with the
called function. Figure 7 contains an example.
-.. comment XXX FIX FORMATTING
-
.. code-block:: c
int
@@ -448,25 +317,23 @@ called function. Figure 7 contains an example.
.L1: call bar
or %g0,%g1,%o7
-Figure 7 - Example of tail call elimination
+*Figure 7 - Example of tail call elimination*
-Note that the call instruction overwrites register %o7 with the program
-counter. Therefore the above code saves the old value of %o7, and restores
-it in the delay slot of the call instruction. If the function call is
-register indirect, this twiddling with %o7 can be avoided, but of course
+Note that the call instruction overwrites register ``%o7`` with the program
+counter. Therefore the above code saves the old value of ``%o7``, and restores
+it in the delay slot of the call instruction. If the function ``call`` is
+register indirect, this twiddling with ``%o7`` can be avoided, but of course
that form of call is slower on modern processors.
The benefit of tail call elimination is to remove an indirection upon
return. It is also needed to reduce register window usage, since otherwise
-the foo() function in Figure 7 would need to allocate a stack frame to
+the ``foo()`` function in Figure 7 would need to allocate a stack frame to
save the program counter.
A special form of tail call elimination is tail recursion elimination,
which detects functions calling themselves, and replaces it with a simple
branch. Figure 8 contains an example.
-.. comment XXX FIX FORMATTING
-
.. code-block:: c
int
@@ -487,7 +354,7 @@ branch. Figure 8 contains an example.
.L1: retl
or %g0,1,%o0
-Figure 8 - Example of tail recursion elimination
+*Figure 8 - Example of tail recursion elimination*
Needless to say, these optimizations produce code that is difficult
to debug.
@@ -504,14 +371,14 @@ When compiling for debugging, compilers will generate additional code
as well as avoid some optimizations in order to allow reconstructing
situations during execution. For example, GCC/GDB makes sure original
parameter values are kept intact somewhere for future parsing of
-the procedure call stack. The live in registers other than %i0 are
-not touched. %i0 itself is copied into a free local register, and its
+the procedure call stack. The live ``in`` registers other than %i0 are
+not touched. %i0 itself is copied into a free ``local`` register, and its
location is noted in the symbol file. (You can find out where variables
reside by using the "info address" command in GDB.)
Given that much of the semantics relating to stack handling and procedure
call entry/exit code is only recommended, debuggers will sometimes
-be fooled. For example, the decision as to wether or not the current
+be fooled. For example, the decision as to whether or not the current
procedure is a leaf one or not can be incorrect. In this case a spurious
procedure will be inserted between the current procedure and it's "real"
parent. Another example is when the application maintains its own implicit
@@ -539,8 +406,6 @@ other than the current one, hence the code does additional save/restore
instructions. It's pretty tricky to understand the code, but figure 1
should be of help.
-.. comment XXX FIX FORMATTING
-
.. code-block:: c
/* a SAVE instruction caused a trap */
@@ -580,7 +445,7 @@ should be of help.
jmp %l1
rett %l2
-Figure 9 - window_underflow trap handler
+*Figure 9 - window_underflow trap handler*
.. code-block:: c
@@ -626,4 +491,5 @@ Figure 9 - window_underflow trap handler
jmp %l1
rett %l2
-Figure 10 - window_underflow trap handler
+*Figure 10 - window_underflow trap handler*
+