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| <h3 class="section">4.4 <samp><span class="command">ld</span></samp> and the ARM family</h3> |
| |
| <p><a name="index-ARM-interworking-support-574"></a><a name="index-g_t_002d_002dsupport_002dold_002dcode-575"></a>For the ARM, <samp><span class="command">ld</span></samp> will generate code stubs to allow functions calls |
| between ARM and Thumb code. These stubs only work with code that has |
| been compiled and assembled with the ‘<samp><span class="samp">-mthumb-interwork</span></samp>’ command |
| line option. If it is necessary to link with old ARM object files or |
| libraries, which have not been compiled with the -mthumb-interwork |
| option then the ‘<samp><span class="samp">--support-old-code</span></samp>’ command line switch should be |
| given to the linker. This will make it generate larger stub functions |
| which will work with non-interworking aware ARM code. Note, however, |
| the linker does not support generating stubs for function calls to |
| non-interworking aware Thumb code. |
| |
| <p><a name="index-thumb-entry-point-576"></a><a name="index-entry-point_002c-thumb-577"></a><a name="index-g_t_002d_002dthumb_002dentry_003d_0040var_007bentry_007d-578"></a>The ‘<samp><span class="samp">--thumb-entry</span></samp>’ switch is a duplicate of the generic |
| ‘<samp><span class="samp">--entry</span></samp>’ switch, in that it sets the program's starting address. |
| But it also sets the bottom bit of the address, so that it can be |
| branched to using a BX instruction, and the program will start |
| executing in Thumb mode straight away. |
| |
| <p><a name="index-PE-import-table-prefixing-579"></a><a name="index-g_t_002d_002duse_002dnul_002dprefixed_002dimport_002dtables-580"></a>The ‘<samp><span class="samp">--use-nul-prefixed-import-tables</span></samp>’ switch is specifying, that |
| the import tables idata4 and idata5 have to be generated with a zero |
| elememt prefix for import libraries. This is the old style to generate |
| import tables. By default this option is turned off. |
| |
| <p><a name="index-BE8-581"></a><a name="index-g_t_002d_002dbe8-582"></a>The ‘<samp><span class="samp">--be8</span></samp>’ switch instructs <samp><span class="command">ld</span></samp> to generate BE8 format |
| executables. This option is only valid when linking big-endian objects. |
| The resulting image will contain big-endian data and little-endian code. |
| |
| <p><a name="index-TARGET1-583"></a><a name="index-g_t_002d_002dtarget1_002drel-584"></a><a name="index-g_t_002d_002dtarget1_002dabs-585"></a>The ‘<samp><span class="samp">R_ARM_TARGET1</span></samp>’ relocation is typically used for entries in the |
| ‘<samp><span class="samp">.init_array</span></samp>’ section. It is interpreted as either ‘<samp><span class="samp">R_ARM_REL32</span></samp>’ |
| or ‘<samp><span class="samp">R_ARM_ABS32</span></samp>’, depending on the target. The ‘<samp><span class="samp">--target1-rel</span></samp>’ |
| and ‘<samp><span class="samp">--target1-abs</span></samp>’ switches override the default. |
| |
| <p><a name="index-TARGET2-586"></a><a name="index-g_t_002d_002dtarget2_003d_0040var_007btype_007d-587"></a>The ‘<samp><span class="samp">--target2=type</span></samp>’ switch overrides the default definition of the |
| ‘<samp><span class="samp">R_ARM_TARGET2</span></samp>’ relocation. Valid values for ‘<samp><span class="samp">type</span></samp>’, their |
| meanings, and target defaults are as follows: |
| <dl> |
| <dt>‘<samp><span class="samp">rel</span></samp>’<dd>‘<samp><span class="samp">R_ARM_REL32</span></samp>’ (arm*-*-elf, arm*-*-eabi) |
| <br><dt>‘<samp><span class="samp">abs</span></samp>’<dd>‘<samp><span class="samp">R_ARM_ABS32</span></samp>’ (arm*-*-symbianelf) |
| <br><dt>‘<samp><span class="samp">got-rel</span></samp>’<dd>‘<samp><span class="samp">R_ARM_GOT_PREL</span></samp>’ (arm*-*-linux, arm*-*-*bsd) |
| </dl> |
| |
| <p><a name="index-FIX_005fV4BX-588"></a><a name="index-g_t_002d_002dfix_002dv4bx-589"></a>The ‘<samp><span class="samp">R_ARM_V4BX</span></samp>’ relocation (defined by the ARM AAELF |
| specification) enables objects compiled for the ARMv4 architecture to be |
| interworking-safe when linked with other objects compiled for ARMv4t, but |
| also allows pure ARMv4 binaries to be built from the same ARMv4 objects. |
| |
| <p>In the latter case, the switch <samp><span class="option">--fix-v4bx</span></samp> must be passed to the |
| linker, which causes v4t <code>BX rM</code> instructions to be rewritten as |
| <code>MOV PC,rM</code>, since v4 processors do not have a <code>BX</code> instruction. |
| |
| <p>In the former case, the switch should not be used, and ‘<samp><span class="samp">R_ARM_V4BX</span></samp>’ |
| relocations are ignored. |
| |
| <p><a name="index-FIX_005fV4BX_005fINTERWORKING-590"></a><a name="index-g_t_002d_002dfix_002dv4bx_002dinterworking-591"></a>Replace <code>BX rM</code> instructions identified by ‘<samp><span class="samp">R_ARM_V4BX</span></samp>’ |
| relocations with a branch to the following veneer: |
| |
| <pre class="smallexample"> TST rM, #1 |
| MOVEQ PC, rM |
| BX Rn |
| </pre> |
| <p>This allows generation of libraries/applications that work on ARMv4 cores |
| and are still interworking safe. Note that the above veneer clobbers the |
| condition flags, so may cause incorrect progrm behavior in rare cases. |
| |
| <p><a name="index-USE_005fBLX-592"></a><a name="index-g_t_002d_002duse_002dblx-593"></a>The ‘<samp><span class="samp">--use-blx</span></samp>’ switch enables the linker to use ARM/Thumb |
| BLX instructions (available on ARMv5t and above) in various |
| situations. Currently it is used to perform calls via the PLT from Thumb |
| code using BLX rather than using BX and a mode-switching stub before |
| each PLT entry. This should lead to such calls executing slightly faster. |
| |
| <p>This option is enabled implicitly for SymbianOS, so there is no need to |
| specify it if you are using that target. |
| |
| <p><a name="index-VFP11_005fDENORM_005fFIX-594"></a><a name="index-g_t_002d_002dvfp11_002ddenorm_002dfix-595"></a>The ‘<samp><span class="samp">--vfp11-denorm-fix</span></samp>’ switch enables a link-time workaround for a |
| bug in certain VFP11 coprocessor hardware, which sometimes allows |
| instructions with denorm operands (which must be handled by support code) |
| to have those operands overwritten by subsequent instructions before |
| the support code can read the intended values. |
| |
| <p>The bug may be avoided in scalar mode if you allow at least one |
| intervening instruction between a VFP11 instruction which uses a register |
| and another instruction which writes to the same register, or at least two |
| intervening instructions if vector mode is in use. The bug only affects |
| full-compliance floating-point mode: you do not need this workaround if |
| you are using "runfast" mode. Please contact ARM for further details. |
| |
| <p>This workaround is enabled for scalar code by default for |
| pre-ARMv7 architectures, but disabled by default for later |
| architectures. If you know you are not using buggy VFP11 hardware, |
| you can disable the workaround by specifying the linker option |
| ‘<samp><span class="samp">--vfp-denorm-fix=none</span></samp>’. If you are using VFP vector mode, you |
| should specify ‘<samp><span class="samp">--vfp-denorm-fix=vector</span></samp>’. |
| |
| <p>If the workaround is enabled, instructions are scanned for |
| potentially-troublesome sequences, and a veneer is created for each |
| such sequence which may trigger the erratum. The veneer consists of the |
| first instruction of the sequence and a branch back to the subsequent |
| instruction. The original instruction is then replaced with a branch to |
| the veneer. The extra cycles required to call and return from the veneer |
| are sufficient to avoid the erratum in both the scalar and vector cases. |
| |
| <p><a name="index-NO_005fENUM_005fSIZE_005fWARNING-596"></a><a name="index-g_t_002d_002dno_002denum_002dsize_002dwarning-597"></a>The <samp><span class="option">--no-enum-size-warning</span></samp> switch prevents the linker from |
| warning when linking object files that specify incompatible EABI |
| enumeration size attributes. For example, with this switch enabled, |
| linking of an object file using 32-bit enumeration values with another |
| using enumeration values fitted into the smallest possible space will |
| not be diagnosed. |
| |
| <p><a name="index-NO_005fWCHAR_005fSIZE_005fWARNING-598"></a><a name="index-g_t_002d_002dno_002dwchar_002dsize_002dwarning-599"></a>The <samp><span class="option">--no-wchar-size-warning</span></samp> switch prevents the linker from |
| warning when linking object files that specify incompatible EABI |
| <code>wchar_t</code> size attributes. For example, with this switch enabled, |
| linking of an object file using 32-bit <code>wchar_t</code> values with another |
| using 16-bit <code>wchar_t</code> values will not be diagnosed. |
| |
| <p><a name="index-PIC_005fVENEER-600"></a><a name="index-g_t_002d_002dpic_002dveneer-601"></a>The ‘<samp><span class="samp">--pic-veneer</span></samp>’ switch makes the linker use PIC sequences for |
| ARM/Thumb interworking veneers, even if the rest of the binary |
| is not PIC. This avoids problems on uClinux targets where |
| ‘<samp><span class="samp">--emit-relocs</span></samp>’ is used to generate relocatable binaries. |
| |
| <p><a name="index-STUB_005fGROUP_005fSIZE-602"></a><a name="index-g_t_002d_002dstub_002dgroup_002dsize_003d_0040var_007bN_007d-603"></a>The linker will automatically generate and insert small sequences of |
| code into a linked ARM ELF executable whenever an attempt is made to |
| perform a function call to a symbol that is too far away. The |
| placement of these sequences of instructions - called stubs - is |
| controlled by the command line option <samp><span class="option">--stub-group-size=N</span></samp>. |
| The placement is important because a poor choice can create a need for |
| duplicate stubs, increasing the code sizw. The linker will try to |
| group stubs together in order to reduce interruptions to the flow of |
| code, but it needs guidance as to how big these groups should be and |
| where they should be placed. |
| |
| <p>The value of ‘<samp><span class="samp">N</span></samp>’, the parameter to the |
| <samp><span class="option">--stub-group-size=</span></samp> option controls where the stub groups are |
| placed. If it is negative then all stubs are placed after the first |
| branch that needs them. If it is positive then the stubs can be |
| placed either before or after the branches that need them. If the |
| value of ‘<samp><span class="samp">N</span></samp>’ is 1 (either +1 or -1) then the linker will choose |
| exactly where to place groups of stubs, using its built in heuristics. |
| A value of ‘<samp><span class="samp">N</span></samp>’ greater than 1 (or smaller than -1) tells the |
| linker that a single group of stubs can service at most ‘<samp><span class="samp">N</span></samp>’ bytes |
| from the input sections. |
| |
| <p>The default, if <samp><span class="option">--stub-group-size=</span></samp> is not specified, is |
| ‘<samp><span class="samp">N = +1</span></samp>’. |
| |
| <p>Farcalls stubs insertion is fully supported for the ARM-EABI target |
| only, because it relies on object files properties not present |
| otherwise. |
| |
| <p><a name="index-Cortex_002dA8-erratum-workaround-604"></a><a name="index-g_t_002d_002dfix_002dcortex_002da8-605"></a><a name="index-g_t_002d_002dno_002dfix_002dcortex_002da8-606"></a>The ‘<samp><span class="samp">--fix-cortex-a8</span></samp>’ switch enables a link-time workaround for an erratum in certain Cortex-A8 processors. The workaround is enabled by default if you are targeting the ARM v7-A architecture profile. It can be enabled otherwise by specifying ‘<samp><span class="samp">--fix-cortex-a8</span></samp>’, or disabled unconditionally by specifying ‘<samp><span class="samp">--no-fix-cortex-a8</span></samp>’. |
| |
| <p>The erratum only affects Thumb-2 code. Please contact ARM for further details. |
| |
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