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| <h5 class="subsubsection">9.1.3.3 Relocations</h5> |
| |
| <p><a name="index-Alpha-relocations-530"></a><a name="index-relocations_002c-Alpha-531"></a> |
| Some of these relocations are available for ECOFF, but mostly |
| only for ELF. They are modeled after the relocation format |
| introduced in Digital Unix 4.0, but there are additions. |
| |
| <p>The format is ‘<samp><span class="samp">!</span><var>tag</var></samp>’ or ‘<samp><span class="samp">!</span><var>tag</var><span class="samp">!</span><var>number</var></samp>’ |
| where <var>tag</var> is the name of the relocation. In some cases |
| <var>number</var> is used to relate specific instructions. |
| |
| <p>The relocation is placed at the end of the instruction like so: |
| |
| <pre class="example"> ldah $0,a($29) !gprelhigh |
| lda $0,a($0) !gprellow |
| ldq $1,b($29) !literal!100 |
| ldl $2,0($1) !lituse_base!100 |
| </pre> |
| <dl> |
| <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 |
| from the GOT. |
| |
| <p>A sequence number <var>N</var> is optional, and if present is used to pair |
| <code>lituse</code> relocations with this <code>literal</code> relocation. The |
| <code>lituse</code> relocations are used by the linker to optimize the code |
| based on the final location of the symbol. |
| |
| <p>Note that these optimizations are dependent on the data flow of the |
| program. Therefore, if <em>any</em> <code>lituse</code> is paired with a |
| <code>literal</code> relocation, then <em>all</em> uses of the register set by |
| the <code>literal</code> instruction must also be marked with <code>lituse</code> |
| relocations. This is because the original <code>literal</code> instruction |
| may be deleted or transformed into another instruction. |
| |
| <p>Also note that there may be a one-to-many relationship between |
| <code>literal</code> and <code>lituse</code>, but not a many-to-one. That is, if |
| there are two code paths that load up the same address and feed the |
| value to a single use, then the use may not use a <code>lituse</code> |
| relocation. |
| |
| <br><dt><code>!lituse_base!</code><var>N</var><dd>Used with any memory format instruction (e.g. <code>ldl</code>) to indicate |
| that the literal is used for an address load. The offset field of the |
| instruction must be zero. During relaxation, the code may be altered |
| to use a gp-relative load. |
| |
| <br><dt><code>!lituse_jsr!</code><var>N</var><dd>Used with a register branch format instruction (e.g. <code>jsr</code>) to |
| indicate that the literal is used for a call. During relaxation, the |
| code may be altered to use a direct branch (e.g. <code>bsr</code>). |
| |
| <br><dt><code>!lituse_jsrdirect!</code><var>N</var><dd>Similar to <code>lituse_jsr</code>, but also that this call cannot be vectored |
| through a PLT entry. This is useful for functions with special calling |
| conventions which do not allow the normal call-clobbered registers to be |
| clobbered. |
| |
| <br><dt><code>!lituse_bytoff!</code><var>N</var><dd>Used with a byte mask instruction (e.g. <code>extbl</code>) to indicate |
| that only the low 3 bits of the address are relevant. During relaxation, |
| the code may be altered to use an immediate instead of a register shift. |
| |
| <br><dt><code>!lituse_addr!</code><var>N</var><dd>Used with any other instruction to indicate that the original address |
| is in fact used, and the original <code>ldq</code> instruction may not be |
| altered or deleted. This is useful in conjunction with <code>lituse_jsr</code> |
| to test whether a weak symbol is defined. |
| |
| <pre class="example"> ldq $27,foo($29) !literal!1 |
| beq $27,is_undef !lituse_addr!1 |
| jsr $26,($27),foo !lituse_jsr!1 |
| </pre> |
| <br><dt><code>!lituse_tlsgd!</code><var>N</var><dd>Used with a register branch format instruction to indicate that the |
| literal is the call to <code>__tls_get_addr</code> used to compute the |
| address of the thread-local storage variable whose descriptor was |
| loaded with <code>!tlsgd!</code><var>N</var>. |
| |
| <br><dt><code>!lituse_tlsldm!</code><var>N</var><dd>Used with a register branch format instruction to indicate that the |
| literal is the call to <code>__tls_get_addr</code> used to compute the |
| address of the base of the thread-local storage block for the current |
| module. The descriptor for the module must have been loaded with |
| <code>!tlsldm!</code><var>N</var>. |
| |
| <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 |
| address, a-la the <code>ldgp</code> macro. The source register for the |
| <code>ldah</code> instruction must contain the address of the <code>ldah</code> |
| instruction. There must be exactly one <code>lda</code> instruction paired |
| with the <code>ldah</code> instruction, though it may appear anywhere in |
| the instruction stream. The immediate operands must be zero. |
| |
| <pre class="example"> bsr $26,foo |
| ldah $29,0($26) !gpdisp!1 |
| lda $29,0($29) !gpdisp!1 |
| </pre> |
| <br><dt><code>!gprelhigh</code><dd>Used with an <code>ldah</code> instruction to add the high 16 bits of a |
| 32-bit displacement from the GP. |
| |
| <br><dt><code>!gprellow</code><dd>Used with any memory format instruction to add the low 16 bits of a |
| 32-bit displacement from the GP. |
| |
| <br><dt><code>!gprel</code><dd>Used with any memory format instruction to add a 16-bit displacement |
| from the GP. |
| |
| <br><dt><code>!samegp</code><dd>Used with any branch format instruction to skip the GP load at the |
| target address. The referenced symbol must have the same GP as the |
| source object file, and it must be declared to either not use <code>$27</code> |
| or perform a standard GP load in the first two instructions via the |
| <code>.prologue</code> directive. |
| |
| <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 |
| descriptor for a symbol in the GOT. |
| |
| <p>The sequence number <var>N</var> is optional, and if present it used to |
| pair the descriptor load with both the <code>literal</code> loading the |
| address of the <code>__tls_get_addr</code> function and the <code>lituse_tlsgd</code> |
| marking the call to that function. |
| |
| <p>For proper relaxation, both the <code>tlsgd</code>, <code>literal</code> and |
| <code>lituse</code> relocations must be in the same extended basic block. |
| That is, the relocation with the lowest address must be executed |
| first at runtime. |
| |
| <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 |
| descriptor for the current module in the GOT. |
| |
| <p>Similar in other respects to <code>tlsgd</code>. |
| |
| <br><dt><code>!gotdtprel</code><dd>Used with an <code>ldq</code> instruction to load the offset of the TLS |
| symbol within its module's thread-local storage block. Also known |
| as the dynamic thread pointer offset or dtp-relative offset. |
| |
| <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. |
| |
| <br><dt><code>!gottprel</code><dd>Used with an <code>ldq</code> instruction to load the offset of the TLS |
| symbol from the thread pointer. Also known as the tp-relative offset. |
| |
| <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. |
| </dl> |
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