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annotate toolchain/binutils-patches/elf32-arm.patch @ 250:3d88461d8284

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beginning of TIFFS IVA documentation
author Michael Spacefalcon <msokolov@ivan.Harhan.ORG>
date Fri, 31 Jan 2014 07:37:39 +0000
parents ebe258a85813
children
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ebe258a85813 toolchain: binutils patch to emit the "short" form of Thumb->ARM call veneers
Michael Spacefalcon <msokolov@ivan.Harhan.ORG>
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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 }