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| <h3 class="section">4.3 <samp><span class="command">ld</span></samp> and the ARM family</h3> |
| |
| <p><a name="index-ARM-interworking-support-629"></a><a name="index-g_t_002d_002dsupport_002dold_002dcode-630"></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-631"></a><a name="index-entry-point_002c-thumb-632"></a><a name="index-g_t_002d_002dthumb_002dentry_003d_0040var_007bentry_007d-633"></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-634"></a><a name="index-g_t_002d_002duse_002dnul_002dprefixed_002dimport_002dtables-635"></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 |
| element prefix for import libraries. This is the old style to generate |
| import tables. By default this option is turned off. |
| |
| <p><a name="index-BE8-636"></a><a name="index-g_t_002d_002dbe8-637"></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 - ie ones which have been assembled with the <samp><span class="option">-EB</span></samp> |
| option. The resulting image will contain big-endian data and |
| little-endian code. |
| |
| <p><a name="index-TARGET1-638"></a><a name="index-g_t_002d_002dtarget1_002drel-639"></a><a name="index-g_t_002d_002dtarget1_002dabs-640"></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-TARGET2-641"></a><a name="index-g_t_002d_002dtarget2_003d_0040var_007btype_007d-642"></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-643"></a><a name="index-g_t_002d_002dfix_002dv4bx-644"></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-645"></a><a name="index-g_t_002d_002dfix_002dv4bx_002dinterworking-646"></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 program behavior in rare cases. |
| |
| <p><a name="index-USE_005fBLX-647"></a><a name="index-g_t_002d_002duse_002dblx-648"></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-649"></a><a name="index-g_t_002d_002dvfp11_002ddenorm_002dfix-650"></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>If you know you are using buggy VFP11 hardware, you can |
| enable this workaround by specifying the linker option |
| ‘<samp><span class="samp">--vfp-denorm-fix=scalar</span></samp>’ if you are using the VFP11 scalar |
| mode only, or ‘<samp><span class="samp">--vfp-denorm-fix=vector</span></samp>’ if you are using |
| vector mode (the latter also works for scalar code). The default is |
| ‘<samp><span class="samp">--vfp-denorm-fix=none</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-ARM1176-erratum-workaround-651"></a><a name="index-g_t_002d_002dfix_002darm1176-652"></a><a name="index-g_t_002d_002dno_002dfix_002darm1176-653"></a>The ‘<samp><span class="samp">--fix-arm1176</span></samp>’ switch enables a link-time workaround for an erratum |
| in certain ARM1176 processors. The workaround is enabled by default if you |
| are targeting ARM v6 (excluding ARM v6T2) or earlier. It can be disabled |
| unconditionally by specifying ‘<samp><span class="samp">--no-fix-arm1176</span></samp>’. |
| |
| <p>Further information is available in the “ARM1176JZ-S and ARM1176JZF-S |
| Programmer Advice Notice” available on the ARM documentation website at: |
| http://infocenter.arm.com/. |
| |
| <p><a name="index-STM32L4xx-erratum-workaround-654"></a><a name="index-g_t_002d_002dfix_002dstm32l4xx_002d629360-655"></a> |
| The ‘<samp><span class="samp">--fix-stm32l4xx-629360</span></samp>’ switch enables a link-time |
| workaround for a bug in the bus matrix / memory controller for some of |
| the STM32 Cortex-M4 based products (STM32L4xx). When accessing |
| off-chip memory via the affected bus for bus reads of 9 words or more, |
| the bus can generate corrupt data and/or abort. These are only |
| core-initiated accesses (not DMA), and might affect any access: |
| integer loads such as LDM, POP and floating-point loads such as VLDM, |
| VPOP. Stores are not affected. |
| |
| <p>The bug can be avoided by splitting memory accesses into the |
| necessary chunks to keep bus reads below 8 words. |
| |
| <p>The workaround is not enabled by default, this is equivalent to use |
| ‘<samp><span class="samp">--fix-stm32l4xx-629360=none</span></samp>’. If you know you are using buggy |
| STM32L4xx hardware, you can enable the workaround by specifying the |
| linker option ‘<samp><span class="samp">--fix-stm32l4xx-629360</span></samp>’, or the equivalent |
| ‘<samp><span class="samp">--fix-stm32l4xx-629360=default</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 in a |
| replacement sequence emulating the behaviour of the original one and a |
| branch back to the subsequent instruction. The original instruction is |
| then replaced with a branch to the veneer. |
| |
| <p>The workaround does not always preserve the memory access order for |
| the LDMDB instruction, when the instruction loads the PC. |
| |
| <p>The workaround is not able to handle problematic instructions when |
| they are in the middle of an IT block, since a branch is not allowed |
| there. In that case, the linker reports a warning and no replacement |
| occurs. |
| |
| <p>The workaround is not able to replace problematic instructions with a |
| PC-relative branch instruction if the ‘<samp><span class="samp">.text</span></samp>’ section is too |
| large. In that case, when the branch that replaces the original code |
| cannot be encoded, the linker reports a warning and no replacement |
| occurs. |
| |
| <p><a name="index-NO_005fENUM_005fSIZE_005fWARNING-656"></a><a name="index-g_t_002d_002dno_002denum_002dsize_002dwarning-657"></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-658"></a><a name="index-g_t_002d_002dno_002dwchar_002dsize_002dwarning-659"></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-660"></a><a name="index-g_t_002d_002dpic_002dveneer-661"></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-662"></a><a name="index-g_t_002d_002dstub_002dgroup_002dsize_003d_0040var_007bN_007d-663"></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 size. 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-664"></a><a name="index-g_t_002d_002dfix_002dcortex_002da8-665"></a><a name="index-g_t_002d_002dno_002dfix_002dcortex_002da8-666"></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. |
| |
| <p><a name="index-Cortex_002dA53-erratum-835769-workaround-667"></a><a name="index-g_t_002d_002dfix_002dcortex_002da53_002d835769-668"></a><a name="index-g_t_002d_002dno_002dfix_002dcortex_002da53_002d835769-669"></a>The ‘<samp><span class="samp">--fix-cortex-a53-835769</span></samp>’ switch enables a link-time workaround for erratum 835769 present on certain early revisions of Cortex-A53 processors. The workaround is disabled by default. It can be enabled by specifying ‘<samp><span class="samp">--fix-cortex-a53-835769</span></samp>’, or disabled unconditionally by specifying ‘<samp><span class="samp">--no-fix-cortex-a53-835769</span></samp>’. |
| |
| <p>Please contact ARM for further details. |
| |
| <p><a name="index-g_t_002d_002dmerge_002dexidx_002dentries-670"></a><a name="index-g_t_002d_002dno_002dmerge_002dexidx_002dentries-671"></a><a name="index-Merging-exidx-entries-672"></a>The ‘<samp><span class="samp">--no-merge-exidx-entries</span></samp>’ switch disables the merging of adjacent exidx entries in debuginfo. |
| |
| <p><a name="index-g_t_002d_002dlong_002dplt-673"></a><a name="index-g_t32_002dbit-PLT-entries-674"></a>The ‘<samp><span class="samp">--long-plt</span></samp>’ option enables the use of 16 byte PLT entries |
| which support up to 4Gb of code. The default is to use 12 byte PLT |
| entries which only support 512Mb of code. |
| |
| <p><a name="index-g_t_002d_002dno_002dapply_002ddynamic_002drelocs-675"></a><a name="index-AArch64-rela-addend-676"></a>The ‘<samp><span class="samp">--no-apply-dynamic-relocs</span></samp>’ option makes AArch64 linker do not apply |
| link-time values for dynamic relocations. |
| |
| <p><a name="index-Placement-of-SG-veneers-677"></a>All SG veneers are placed in the special output section <code>.gnu.sgstubs</code>. |
| Its start address must be set, either with the command-line option |
| ‘<samp><span class="samp">--section-start</span></samp>’ or in a linker script, to indicate where to place these |
| veneers in memory. |
| |
| <p><a name="index-g_t_002d_002dcmse_002dimplib-678"></a><a name="index-Secure-gateway-import-library-679"></a>The ‘<samp><span class="samp">--cmse-implib</span></samp>’ option requests that the import libraries |
| specified by the ‘<samp><span class="samp">--out-implib</span></samp>’ and ‘<samp><span class="samp">--in-implib</span></samp>’ options are |
| secure gateway import libraries, suitable for linking a non-secure |
| executable against secure code as per ARMv8-M Security Extensions. |
| |
| <p><a name="index-g_t_002d_002din_002dimplib_003d_0040var_007bfile_007d-680"></a><a name="index-Input-import-library-681"></a>The ‘<samp><span class="samp">--in-implib=file</span></samp>’ specifies an input import library whose symbols |
| must keep the same address in the executable being produced. A warning is |
| given if no ‘<samp><span class="samp">--out-implib</span></samp>’ is given but new symbols have been introduced |
| in the executable that should be listed in its import library. Otherwise, if |
| ‘<samp><span class="samp">--out-implib</span></samp>’ is specified, the symbols are added to the output import |
| library. A warning is also given if some symbols present in the input import |
| library have disappeared from the executable. This option is only effective |
| for Secure Gateway import libraries, ie. when ‘<samp><span class="samp">--cmse-implib</span></samp>’ is |
| specified. |
| |
| </body></html> |
| |
