Jim Laskey
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@ -84,6 +84,13 @@
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<li><a href="#targetimpls">Target-specific Implementation Notes</a>
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<ul>
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<li><a href="#x86">The X86 backend</a></li>
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<li><a href="#ppc">The PowerPC backend</a></li>
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<ul>
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<li><a href="#ppc_abi">LLVM PowerPC ABI</a></li>
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<li><a href="#ppc_frame">Frame Layout</a></li>
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<li><a href="#ppc_prolog">Prolog/Epilog</a></li>
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<li><a href="#ppc_dynamic">Dynamic Allocation</a></li>
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</ul>
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</ul>
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</li>
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@ -1723,6 +1730,221 @@ a character per operand with an optional special size. For example:</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="ppc">The PowerPC backend</a>
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</div>
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<div class="doc_text">
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<p>The PowerPC code generator lives in the lib/Target/PowerPC directory. The
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code generation is retargetable to several variations or <i>subtargets</i> of
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the PowerPC ISA; including ppc32, ppc64 and altivec.
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="ppc_abi">LLVM PowerPC ABI</a>
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</div>
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<div class="doc_text">
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<p>LLVM follows the AIX PowerPC ABI, with two deviations. LLVM uses a PC
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relative (PIC) or static addressing for accessing global values, so no TOC (r2)
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is used. Second, r31 is used as a frame pointer to allow dynamic growth of a
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stack frame. LLVM takes advantage of having no TOC to provide space to save
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the frame pointer in the PowerPC linkage area of the caller frame. Other
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details of PowerPC ABI can be found at <a
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href="http://developer.apple.com/documentation/DeveloperTools/Conceptual/
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LowLevelABI/Articles/32bitPowerPC.html" target="_blank">PowerPC ABI.</a> Note:
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This link describes the 32 bit ABI. The 64 bit ABI is similar except space for
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GPRs are 8 bytes wide (not 4) and r13 is reserved for system use.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="ppc_frame">Frame Layout</a>
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</div>
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<div class="doc_text">
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<p>The size of a PowerPC frame is usually fixed for the duration of a
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function's invocation. Since the frame is fixed size, all references into
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the frame can be accessed via fixed offsets from the stack pointer. The
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exception to this is when dynamic alloca or variable sized arrays are present,
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then a base pointer (r31) is used as a proxy for the stack pointer and stack
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pointer is free to grow or shrink. A base pointer is also used if llvm-gcc is
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not passed the -fomit-frame-pointer flag. The stack pointer is always aligned to
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16 bytes, so that space allocated for altivec vectors will be properly
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aligned.</p>
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<p>An invocation frame is layed out as follows (low memory at top);</p>
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</div>
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<div class="doc_text">
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<table class="layout">
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<tr>
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<td>Linkage<br><br></td>
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</tr>
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<tr>
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<td>Parameter area<br><br></td>
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</tr>
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<tr>
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<td>Dynamic area<br><br></td>
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</tr>
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<tr>
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<td>Locals area<br><br></td>
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</tr>
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<tr>
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<td>Saved registers area<br><br></td>
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</tr>
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<tr style="border-style: none hidden none hidden;">
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<td><br></td>
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</tr>
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<tr>
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<td>Previous Frame<br><br></td>
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</tr>
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</table>
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</div>
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<div class="doc_text">
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<p>The <i>linkage</i> area is used by a callee to save special registers prior
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to allocating its own frame. Only three entries are relevant to LLVM. The
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first entry is the previous stack pointer (sp), aka link. This allows probing
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tools like gdb or exception handlers to quickly scan the frames in the stack. A
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function epilog can also use the link to pop the frame from the stack. The
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third entry in the linkage area is used to save the return address from the lr
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register. Finally, as mentioned above, the last entry is used to save the
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previous frame pointer (r31.) The entries in the linkage area are the size of a
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GPR, thus the linkage area is 24 bytes long in 32 bit mode and 48 bytes in 64
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bit mode.</p>
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</div>
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<div class="doc_text">
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<p>32 bit linkage area</p>
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<table class="layout">
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<tr>
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<td>0</td>
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<td>Saved SP (r1)</td>
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</tr>
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<tr>
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<td>4</td>
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<td>Saved CR</td>
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</tr>
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<tr>
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<td>8</td>
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<td>Saved LR</td>
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</tr>
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<tr>
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<td>12</td>
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<td>Reserved</td>
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</tr>
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<tr>
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<td>16</td>
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<td>Reserved</td>
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</tr>
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<tr>
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<td>20</td>
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<td>Saved FP (r31)</td>
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</tr>
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</table>
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</div>
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<div class="doc_text">
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<p>64 bit linkage area</p>
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<table class="layout">
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<tr>
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<td>0</td>
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<td>Saved SP (r1)</td>
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</tr>
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<tr>
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<td>8</td>
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<td>Saved CR</td>
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</tr>
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<tr>
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<td>16</td>
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<td>Saved LR</td>
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</tr>
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<tr>
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<td>24</td>
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<td>Reserved</td>
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</tr>
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<tr>
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<td>32</td>
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<td>Reserved</td>
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</tr>
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<tr>
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<td>40</td>
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<td>Saved FP (r31)</td>
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</tr>
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</table>
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</div>
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<div class="doc_text">
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<p>The <i>parameter area</i> is used to store arguments being passed to a callee
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function. Following the PowerPC ABI, the first few arguments are actually
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passed in registers, with the space in the parameter area unused. However, if
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there are not enough registers or the callee is a thunk or vararg function,
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these register arguments can be spilled into the parameter area. Thus, the
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parameter area must be large enough to store all the parameters for the largest
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call sequence made by the caller. The size must also be mimimally large enough
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to spill registers r3-r10. This allows callees blind to the call signature,
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such as thunks and vararg functions, enough space to cache the argument
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registers. Therefore, the parameter area is minimally 32 bytes (64 bytes in 64
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bit mode.) Also note that since the parameter area is a fixed offset from the
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top of the frame, that a callee can access its spilt arguments using fixed
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offsets from the stack pointer (or base pointer.)</p>
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</div>
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<div class="doc_text">
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<p>Combining the information about the linkage, parameter areas and alignment. A
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stack frame is minimally 64 bytes in 32 bit mode and 128 bytes in 64 bit
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mode.</p>
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</div>
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<div class="doc_text">
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<p>The <i>dynamic area</i> starts out as size zero. If a function uses dynamic
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alloca then space is added to the stack, the linkage and parameter areas are
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shifted to top of stack, and the new space is available immediately below the
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linkage and parameter areas. The cost of shifting the linkage and parameter
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areas is minor since only the link value needs to be copied. The link value can
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be easily fetched by adding the original frame size to the base pointer. Note
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that allocations in the dynamic space need to observe 16 byte aligment.</p>
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</div>
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<div class="doc_text">
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<p>The <i>locals area</i> is where the llvm compiler reserves space for local
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variables.</p>
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</div>
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<div class="doc_text">
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<p>The <i>saved registers area</i> is where the llvm compiler spills callee saved
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registers on entry to the callee.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="ppc_prolog">Prolog/Epilog</a>
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</div>
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<div class="doc_text">
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<p>The llvm prolog and epilog are the same as described in the PowerPC ABI, with
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the following exceptions. Callee saved registers are spilled after the frame is
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created. This allows the llvm epilog/prolog support to be common with other
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targets. The base pointer callee saved register r31 is saved in the TOC slot of
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linkage area. This simplifies allocation of space for the base pointer and
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makes it convenient to locate programatically and during debugging.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="ppc_dynamic">Dynamic Allocation</a>
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</div>
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<div class="doc_text">
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<p></p>
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</div>
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<i>TODO - More to come.</i>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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