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annotate docs/readme-4.0.txt @ 418:3832a68d83ef
Fix internal compiler error on "var2 = var1 + 1" patterns
This appears to be the correct fix. It was provided by Tormod Volden
(debian.tormod@gmail.com). The final commit is substantially different from
Tormod's submission mostly due to housecleaning (removing the old patches
and updating the README). Tormod's comments follow.
The original addhi_mem_1 "insn" instruction pattern /matches/ two
memory operands, just with the /constraint/ that these are the same
location. A pattern match tells the compiler "you should be able to use
this, but you might have to work on it to meet the constraints". For
typical constraints on registers the compiler can add "reloads", moving
stuff between registers or from memory, until the constraints are met
and the instruction can be used. However, in this case, no amount of
reloads can make two memory locations the same if they already weren't,
so the compiler breaks down and cries "unable to generate reloads".
It seems this issue only appears if optimization is enabled. The proof
is in gcc's reload.c and is left as an exercise to the reader.
Limiting the matching pattern to identical memory operands avoids
these situations, while allowing the common "var++" cases.
References:
The pattern/constraints difference is explained in
https://gcc.gnu.org/onlinedocs/gccint/Simple-Constraints.html#index-other-register-constraints-3335
author | William Astle <lost@l-w.ca> |
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date | Tue, 29 Mar 2016 21:21:49 -0600 |
parents | 2c24602be78f |
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1 With LWTOOLS 4.0, a substantial reorganization of the project has occurred. |
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2 This document serves to explain the reasoning behind the various changes. |
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3 |
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4 The most obvious change is that the gnu auto tools have been eliminated. |
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5 While they proved useful for initial distribution of the software, |
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6 particularly for construction of the win32 binaries, they have since proved |
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7 to add an unacceptable level of complexity to every aspect of development |
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8 from merely tinkering with source files to doing complete releases. Thus, |
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9 the auto tools have been ditched in favour of specific hand tuned help where |
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10 required. |
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11 |
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12 The other substantial change is that the source code repository has been |
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13 recreated from scratch. The old repository was full of cruft from various |
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14 revision control systems that were used over the years (CVS, Subversion, and |
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15 Mercurial). It was felt that starting a new Mercurial repository with a |
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16 completely clean slate would simplify matters substantially. Thus, the old |
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17 repository now serves as an archive. |