Hello,
I'm using Linaro GCC 4.9-2015.01-3 for AArch64, and trying to disable the generation of guarded instructions. More specifically, I'd like not to see instructions such as 'cset', 'csinc', 'ccmp', 'fccmp', etc.
So far, it seemed that the flag '-fno-if-conversion2' could do the job, but I still see those instructions in my disassembled program functions (I guess that it may not be possible to remove them from standard libs).
My command line: aarch64-linux-gnu-gcc -static -march=armv8-a -O3 -fno-strict-aliasing -fno-if-conversion2
I also tried '-fno-if-conversion' and both '-fno-if-conversion -fno-if-conversion2' without success.
Regards,
-- Fernando A. Endo, Post-doc
INRIA Rennes-Bretagne Atlantique France
On Tue, Sep 20, 2016 at 8:07 AM, Fernando Endo fernando.endo2@gmail.com wrote:
I'm using Linaro GCC 4.9-2015.01-3 for AArch64, and trying to disable the generation of guarded instructions. More specifically, I'd like not to see instructions such as 'cset', 'csinc', 'ccmp', 'fccmp', etc.
There are a lot of different optimization passes that can generate these instructions, and probably not all of them can be disabled via options. A better approach would be to disable the patterns that generate the conditional move instructions. The compiler should still work without them. In the gcc/config/aarch64/aarch64.md file, you want to disable all of the patterns after the comment
;; ------------------------------------------------------------------- ;; Store-flag and conditional select insns ;; -------------------------------------------------------------------
and before the Logical operations comment. You can disable a pattern by modifying the first set of quotes from "" to "0", or if there is already code there, change it to "0 && ..." where ... is the original code. Or alternatively, you can just delete all of the patterns. I see that there is one crc pattern mixed in among the csel patterns, which is an odd place for it. That one should not be touched, though it is only used if the code being compiled calls one of the crc builtin functions, which is uncommon.
For the conditional compare instructions, this is a little more complicated. You need to disable or delete the cbranchcc4, ccmp<mode>, fccmp<mode>, and fccmpe<mode> patterns in the aarch64.md file. And you also need to delete the #undef and #define in aarch64.c for TARGET_GEN_CCMP_FIRST and TARGET_GEN_CCMP_NEXT.
If you want a more useful patch, you can add a compiler option, and then make all of the patterns conditional on the compiler option, so that they can be turned off and on via the command line instead of needing two different compilers.
I haven't tried this myself, so there could be some issues that I overlooked. I would expect this to work though.
Jim
Hello again,
Thanks for both replies, they'll be useful to me. I wanted to disable conditional execution for research purposes.
Regards,
-- Fernando A. Endo, Post-doc
INRIA Rennes-Bretagne Atlantique France
2016-09-20 20:21 GMT+02:00 Jim Wilson jim.wilson@linaro.org:
On Tue, Sep 20, 2016 at 8:07 AM, Fernando Endo fernando.endo2@gmail.com wrote:
I'm using Linaro GCC 4.9-2015.01-3 for AArch64, and trying to disable the generation of guarded instructions. More specifically, I'd like not to
see
instructions such as 'cset', 'csinc', 'ccmp', 'fccmp', etc.
There are a lot of different optimization passes that can generate these instructions, and probably not all of them can be disabled via options. A better approach would be to disable the patterns that generate the conditional move instructions. The compiler should still work without them. In the gcc/config/aarch64/aarch64.md file, you want to disable all of the patterns after the comment
;; ------------------------------------------------------------------- ;; Store-flag and conditional select insns ;; -------------------------------------------------------------------
and before the Logical operations comment. You can disable a pattern by modifying the first set of quotes from "" to "0", or if there is already code there, change it to "0 && ..." where ... is the original code. Or alternatively, you can just delete all of the patterns. I see that there is one crc pattern mixed in among the csel patterns, which is an odd place for it. That one should not be touched, though it is only used if the code being compiled calls one of the crc builtin functions, which is uncommon.
For the conditional compare instructions, this is a little more complicated. You need to disable or delete the cbranchcc4, ccmp<mode>, fccmp<mode>, and fccmpe<mode> patterns in the aarch64.md file. And you also need to delete the #undef and #define in aarch64.c for TARGET_GEN_CCMP_FIRST and TARGET_GEN_CCMP_NEXT.
If you want a more useful patch, you can add a compiler option, and then make all of the patterns conditional on the compiler option, so that they can be turned off and on via the command line instead of needing two different compilers.
I haven't tried this myself, so there could be some issues that I overlooked. I would expect this to work though.
Jim
linaro-toolchain@lists.linaro.org
-
Fernando Endo -
Jim Wilson