FreeCalypso > hg > freecalypso-tools
annotate toolchain/binutils-patches/elf32-arm.patch @ 0:e7502631a0f9
initial import from freecalypso-sw rev 1033:5ab737ac3ad7
author | Mychaela Falconia <falcon@freecalypso.org> |
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date | Sat, 11 Jun 2016 00:13:35 +0000 |
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1 # The present patch to the ARM ELF linking code in binutils (made against |
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2 # binutils-2.21.1a) is a hack that affects the generation of Thumb->ARM |
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3 # call veneers. The binutils linker can make these veneers in two forms: |
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4 # |
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5 # "Short" "Long" |
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6 # (16-bit) bx pc (16-bit) bx pc |
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7 # (16-bit) nop (16-bit) nop |
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8 # (32-bit) b dest (32-bit) ldr pc, [pc, #-4] |
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9 # (32-bit) .long target |
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10 # |
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11 # For code that's going to run on the Calypso ARM7 processor, the |
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12 # "short" form is the one we want: because of the way the address space |
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13 # is laid out, the range of an ARM branch instruction will always be |
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14 # sufficient for us, whereas the long version is more costly in terms |
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15 # of the number of cycles required, especially when executing from |
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16 # external memory. |
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17 # |
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18 # Unfortunately, the code which decides between these two forms |
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19 # (located in the arm_type_of_stub() function in bfd/elf32-arm.c) |
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20 # is broken. The original code (which you can see below in the |
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21 # "before" part of the patch hunk) exhibits two problems: |
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22 # |
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23 # 1. The distance that matters here is not from the original |
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24 # call point to the destination, but from the stub to |
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25 # the destination - and in this function we don't know |
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26 # where the stub will end up. |
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27 # |
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28 # 2. Because it's an ARM branch, the limits should be |
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29 # ARM_MAX_[BF]WD_BRANCH_OFFSET, not the THM_ ones. |
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30 # |
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31 # The 2nd problem would be trivial to fix, but the 1st one |
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32 # much less so: this function doesn't know where the stub |
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33 # will end up, and considering that for the present purpose |
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34 # of the FreeCalypso project always emitting the "short" |
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35 # version would be perfect, there is no justification for |
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36 # expending the time and effort to restructure the linker |
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37 # to do the proper relaxation for the case at hand. |
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38 # |
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39 # Therefore, the patch we apply is a hack: we simply force the |
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40 # use of the "short" form unconditionally. |
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41 |
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42 *** elf32-arm.c Wed May 11 07:29:12 2011 |
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43 --- elf32-arm-patched.c Mon Aug 19 03:24:38 2013 |
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44 *************** |
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45 *** 3203,3211 **** |
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46 : arm_stub_long_branch_v4t_thumb_arm); |
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47 |
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48 /* Handle v4t short branches. */ |
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49 ! if ((stub_type == arm_stub_long_branch_v4t_thumb_arm) |
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50 ! && (branch_offset <= THM_MAX_FWD_BRANCH_OFFSET) |
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51 ! && (branch_offset >= THM_MAX_BWD_BRANCH_OFFSET)) |
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52 stub_type = arm_stub_short_branch_v4t_thumb_arm; |
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53 } |
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54 } |
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55 --- 3203,3209 ---- |
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56 : arm_stub_long_branch_v4t_thumb_arm); |
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57 |
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58 /* Handle v4t short branches. */ |
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59 ! if (stub_type == arm_stub_long_branch_v4t_thumb_arm) |
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60 stub_type = arm_stub_short_branch_v4t_thumb_arm; |
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61 } |
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62 } |