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According to intel SDM manual, not all compare flag-modifying instructions are marcro-fusible with subsequent jcc instructions. For those non-fusible instructions, -malign-branch doesn't need to align them, only jcc itself needs to be aligned. Here are 2 restrictions which separate macro-fusible instruction from not Restriction 1: If TEST/AND/CMP/ADD/SUB/INC/DEC is one of the following format: cmp m, imm add m, imm sub m, imm test m, imm and m, imm inc m dec m it is unfusible with any jcc instruction. Restriction 2: /* Table 3-2. Macro-Fusible Instructions in Haswell Microarchitecture Note it also works for Skylake and Cascadelake. --------------------------------------------------------------------- | JCC | ADD/SUB/CMP | INC/DEC | TEST/AND | | ------ | ----------- | ------- | -------- | | Jo | N | N | Y | | Jno | N | N | Y | | Jc/Jb | Y | N | Y | | Jae/Jnb | Y | N | Y | | Je/Jz | Y | Y | Y | | Jne/Jnz | Y | Y | Y | | Jna/Jbe | Y | N | Y | | Ja/Jnbe | Y | N | Y | | Js | N | N | Y | | Jns | N | N | Y | | Jp/Jpe | N | N | Y | | Jnp/Jpo | N | N | Y | | Jl/Jnge | Y | Y | Y | | Jge/Jnl | Y | Y | Y | | Jle/Jng | Y | Y | Y | | Jg/Jnle | Y | Y | Y | Update maybe_fused_with_jcc_p to check if operands of CMP like instructions can be fused with condition jump. * gas/config/tc-i386.h (i386_tc_frag_data): Add member mf_type. (TC_FRAG_INIT): Init mf_type. * gas/config/tc-i386.c (enum mf_jcc_kind): New enum. (enum mf_cmp_kind): Ditto. (maybe_fused_with_jcc_p): Add argument mf_cmp_p to get mf_type of corresponding instructons, exclude unfusible instructions. (add_fused_jcc_padding_frag_p): Likewise. (add_branch_padding_frag_p): Likewise. (output_insn): Record mf_type for corresponding instructions. (i386_macro_fusible_p): New function. (i386_next_fusible_jcc_frag): Rename from i386_next_jcc_frag, add argument cmp_fragP to return next fusible jcc frag only. (i386_classify_machine_dependant_frag): Seperate macro-fusible instructions from condition jump. * gas/testsuite/gas/i386/align-branch-9.s: New file. * gas/testsuite/gas/i386/align-branch-9.d: Ditto. * gas/testsuite/gas/i386/x86-64-align-branch-9.s: Ditto. * gas/testsuite/gas/i386/x86-64-align-branch-9.d: Ditto. * gas/testsuite/gas/i386/i386.exp: Run new tests.
README for GNU development tools This directory contains various GNU compilers, assemblers, linkers, debuggers, etc., plus their support routines, definitions, and documentation. If you are receiving this as part of a GDB release, see the file gdb/README. If with a binutils release, see binutils/README; if with a libg++ release, see libg++/README, etc. That'll give you info about this package -- supported targets, how to use it, how to report bugs, etc. It is now possible to automatically configure and build a variety of tools with one command. To build all of the tools contained herein, run the ``configure'' script here, e.g.: ./configure make To install them (by default in /usr/local/bin, /usr/local/lib, etc), then do: make install (If the configure script can't determine your type of computer, give it the name as an argument, for instance ``./configure sun4''. You can use the script ``config.sub'' to test whether a name is recognized; if it is, config.sub translates it to a triplet specifying CPU, vendor, and OS.) If you have more than one compiler on your system, it is often best to explicitly set CC in the environment before running configure, and to also set CC when running make. For example (assuming sh/bash/ksh): CC=gcc ./configure make A similar example using csh: setenv CC gcc ./configure make Much of the code and documentation enclosed is copyright by the Free Software Foundation, Inc. See the file COPYING or COPYING.LIB in the various directories, for a description of the GNU General Public License terms under which you can copy the files. REPORTING BUGS: Again, see gdb/README, binutils/README, etc., for info on where and how to report problems.