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180 lines
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<title>Alpha-Relocs - Using as</title>
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<p>
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<a name="Alpha_002dRelocs"></a>Previous: <a rel="previous" accesskey="p" href="Alpha_002dRegs.html#Alpha_002dRegs">Alpha-Regs</a>,
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Up: <a rel="up" accesskey="u" href="Alpha-Syntax.html#Alpha-Syntax">Alpha Syntax</a>
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<hr><br>
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<h5 class="subsubsection">9.1.3.3 Relocations</h5>
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<p><a name="index-Alpha-relocations-527"></a><a name="index-relocations_002c-Alpha-528"></a>
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Some of these relocations are available for ECOFF, but mostly
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only for ELF. They are modeled after the relocation format
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introduced in Digital Unix 4.0, but there are additions.
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<p>The format is <span class="samp">!</span><var>tag</var> or <span class="samp">!</span><var>tag</var><span class="samp">!</span><var>number</var>
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where <var>tag</var> is the name of the relocation. In some cases
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<var>number</var> is used to relate specific instructions.
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<p>The relocation is placed at the end of the instruction like so:
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<pre class="example"> ldah $0,a($29) !gprelhigh
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lda $0,a($0) !gprellow
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ldq $1,b($29) !literal!100
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ldl $2,0($1) !lituse_base!100
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</pre>
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<dl>
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<dt><code>!literal</code><dt><code>!literal!</code><var>N</var><dd>Used with an <code>ldq</code> instruction to load the address of a symbol
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from the GOT.
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<p>A sequence number <var>N</var> is optional, and if present is used to pair
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<code>lituse</code> relocations with this <code>literal</code> relocation. The
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<code>lituse</code> relocations are used by the linker to optimize the code
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based on the final location of the symbol.
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<p>Note that these optimizations are dependent on the data flow of the
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program. Therefore, if <em>any</em> <code>lituse</code> is paired with a
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<code>literal</code> relocation, then <em>all</em> uses of the register set by
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the <code>literal</code> instruction must also be marked with <code>lituse</code>
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relocations. This is because the original <code>literal</code> instruction
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may be deleted or transformed into another instruction.
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<p>Also note that there may be a one-to-many relationship between
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<code>literal</code> and <code>lituse</code>, but not a many-to-one. That is, if
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there are two code paths that load up the same address and feed the
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value to a single use, then the use may not use a <code>lituse</code>
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relocation.
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<br><dt><code>!lituse_base!</code><var>N</var><dd>Used with any memory format instruction (e.g. <code>ldl</code>) to indicate
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that the literal is used for an address load. The offset field of the
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instruction must be zero. During relaxation, the code may be altered
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to use a gp-relative load.
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<br><dt><code>!lituse_jsr!</code><var>N</var><dd>Used with a register branch format instruction (e.g. <code>jsr</code>) to
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indicate that the literal is used for a call. During relaxation, the
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code may be altered to use a direct branch (e.g. <code>bsr</code>).
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<br><dt><code>!lituse_jsrdirect!</code><var>N</var><dd>Similar to <code>lituse_jsr</code>, but also that this call cannot be vectored
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through a PLT entry. This is useful for functions with special calling
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conventions which do not allow the normal call-clobbered registers to be
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clobbered.
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<br><dt><code>!lituse_bytoff!</code><var>N</var><dd>Used with a byte mask instruction (e.g. <code>extbl</code>) to indicate
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that only the low 3 bits of the address are relevant. During relaxation,
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the code may be altered to use an immediate instead of a register shift.
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<br><dt><code>!lituse_addr!</code><var>N</var><dd>Used with any other instruction to indicate that the original address
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is in fact used, and the original <code>ldq</code> instruction may not be
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altered or deleted. This is useful in conjunction with <code>lituse_jsr</code>
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to test whether a weak symbol is defined.
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<pre class="example"> ldq $27,foo($29) !literal!1
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beq $27,is_undef !lituse_addr!1
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jsr $26,($27),foo !lituse_jsr!1
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</pre>
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<br><dt><code>!lituse_tlsgd!</code><var>N</var><dd>Used with a register branch format instruction to indicate that the
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literal is the call to <code>__tls_get_addr</code> used to compute the
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address of the thread-local storage variable whose descriptor was
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loaded with <code>!tlsgd!</code><var>N</var>.
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<br><dt><code>!lituse_tlsldm!</code><var>N</var><dd>Used with a register branch format instruction to indicate that the
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literal is the call to <code>__tls_get_addr</code> used to compute the
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address of the base of the thread-local storage block for the current
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module. The descriptor for the module must have been loaded with
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<code>!tlsldm!</code><var>N</var>.
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<br><dt><code>!gpdisp!</code><var>N</var><dd>Used with <code>ldah</code> and <code>lda</code> to load the GP from the current
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address, a-la the <code>ldgp</code> macro. The source register for the
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<code>ldah</code> instruction must contain the address of the <code>ldah</code>
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instruction. There must be exactly one <code>lda</code> instruction paired
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with the <code>ldah</code> instruction, though it may appear anywhere in
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the instruction stream. The immediate operands must be zero.
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<pre class="example"> bsr $26,foo
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ldah $29,0($26) !gpdisp!1
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lda $29,0($29) !gpdisp!1
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</pre>
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<br><dt><code>!gprelhigh</code><dd>Used with an <code>ldah</code> instruction to add the high 16 bits of a
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32-bit displacement from the GP.
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<br><dt><code>!gprellow</code><dd>Used with any memory format instruction to add the low 16 bits of a
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32-bit displacement from the GP.
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<br><dt><code>!gprel</code><dd>Used with any memory format instruction to add a 16-bit displacement
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from the GP.
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<br><dt><code>!samegp</code><dd>Used with any branch format instruction to skip the GP load at the
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target address. The referenced symbol must have the same GP as the
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source object file, and it must be declared to either not use <code>$27</code>
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or perform a standard GP load in the first two instructions via the
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<code>.prologue</code> directive.
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<br><dt><code>!tlsgd</code><dt><code>!tlsgd!</code><var>N</var><dd>Used with an <code>lda</code> instruction to load the address of a TLS
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descriptor for a symbol in the GOT.
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<p>The sequence number <var>N</var> is optional, and if present it used to
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pair the descriptor load with both the <code>literal</code> loading the
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address of the <code>__tls_get_addr</code> function and the <code>lituse_tlsgd</code>
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marking the call to that function.
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<p>For proper relaxation, both the <code>tlsgd</code>, <code>literal</code> and
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<code>lituse</code> relocations must be in the same extended basic block.
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That is, the relocation with the lowest address must be executed
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first at runtime.
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<br><dt><code>!tlsldm</code><dt><code>!tlsldm!</code><var>N</var><dd>Used with an <code>lda</code> instruction to load the address of a TLS
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descriptor for the current module in the GOT.
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<p>Similar in other respects to <code>tlsgd</code>.
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<br><dt><code>!gotdtprel</code><dd>Used with an <code>ldq</code> instruction to load the offset of the TLS
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symbol within its module's thread-local storage block. Also known
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as the dynamic thread pointer offset or dtp-relative offset.
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<br><dt><code>!dtprelhi</code><dt><code>!dtprello</code><dt><code>!dtprel</code><dd>Like <code>gprel</code> relocations except they compute dtp-relative offsets.
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<br><dt><code>!gottprel</code><dd>Used with an <code>ldq</code> instruction to load the offset of the TLS
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symbol from the thread pointer. Also known as the tp-relative offset.
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<br><dt><code>!tprelhi</code><dt><code>!tprello</code><dt><code>!tprel</code><dd>Like <code>gprel</code> relocations except they compute tp-relative offsets.
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</dl>
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