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Diffstat (limited to 'gas/doc/c-mips.texi')
| -rw-r--r-- | gas/doc/c-mips.texi | 31 |
1 files changed, 31 insertions, 0 deletions
diff --git a/gas/doc/c-mips.texi b/gas/doc/c-mips.texi index 04666f4..d9a09f1 100644 --- a/gas/doc/c-mips.texi +++ b/gas/doc/c-mips.texi @@ -472,6 +472,37 @@ Also no MIPS16 branches are ever relaxed. By default @samp{--no-relax-branch} is selected, causing any out-of-range branches to produce an error. +@item -mignore-branch-isa +@itemx -mno-ignore-branch-isa +Ignore branch checks for invalid transitions between ISA modes. + +The semantics of branches does not provide for an ISA mode switch, so in +most cases the ISA mode a branch has been encoded for has to be the same +as the ISA mode of the branch's target label. If the ISA modes do not +match, then such a branch, if taken, will cause the ISA mode to remain +unchanged and instructions that follow will be executed in the wrong ISA +mode causing the program to misbehave or crash. + +In the case of the @code{BAL} instruction it may be possible to relax +it to an equivalent @code{JALX} instruction so that the ISA mode is +switched at the run time as required. For other branches no relaxation +is possible and therefore GAS has checks implemented that verify in +branch assembly that the two ISA modes match, and report an error +otherwise so that the problem with code can be diagnosed at the assembly +time rather than at the run time. + +However some assembly code, including generated code produced by some +versions of GCC, may incorrectly include branches to data labels, which +appear to require a mode switch but are either dead or immediately +followed by valid instructions encoded for the same ISA the branch has +been encoded for. While not strictly correct at the source level such +code will execute as intended, so to help with these cases +@samp{-mignore-branch-isa} is supported which disables ISA mode checks +for branches. + +By default @samp{-mno-ignore-branch-isa} is selected, causing any invalid +branch requiring a transition between ISA modes to produce an error. + @cindex @option{-mnan=} command line option, MIPS @item -mnan=@var{encoding} This option indicates whether the source code uses the IEEE 2008 |