| Message ID | 56BD0419.9090702@gmail.com |
|---|---|
| State | New |
| Headers | show |
Ping: https://gcc.gnu.org/ml/gcc-patches/2016-02/msg00809.html On 02/11/2016 02:58 PM, Martin Sebor wrote: > The more than decennnial rtl-optimization/19705 - -fno-branch-count-reg > doesn't prevent decrement and branch instructions on a count register > points out that the documentation of the option leads one to expect > that it prevents the decrement and branch instruction from appearing > in the instruction stream. This isn't the case The option prevents > a dedicated pass from running that introduces such instructions, but > it doesn't prevent other passes from introducing it. The attached > updates the documentation to clarify this. > > Martin
On 02/18/2016 08:39 PM, Martin Sebor wrote: > Ping: > > https://gcc.gnu.org/ml/gcc-patches/2016-02/msg00809.html > > On 02/11/2016 02:58 PM, Martin Sebor wrote: >> The more than decennnial rtl-optimization/19705 - -fno-branch-count-reg >> doesn't prevent decrement and branch instructions on a count register >> points out that the documentation of the option leads one to expect >> that it prevents the decrement and branch instruction from appearing >> in the instruction stream. This isn't the case The option prevents >> a dedicated pass from running that introduces such instructions, but >> it doesn't prevent other passes from introducing it. The attached >> updates the documentation to clarify this I'm not even sure why we have this option to begin with. One could easily argue that it's mis-named or should just go away. In the mean time, your patch at least clarifies what it really does, so it's fine for the trunk. jeff
PR rtl-optimization/19705 - -fno-branch-count-reg doesn't prevent decrement and branch instructions on a count register gcc/ChangeLog: 2016-02-11 Martin Sebor <msebor@redhat.com> PR rtl-optimization/19705 * doc/invoke.texi (Options That Control Optimization): Clarify -fno-branch-count-reg. Index: gcc/doc/invoke.texi =================================================================== --- gcc/doc/invoke.texi (revision 233355) +++ gcc/doc/invoke.texi (working copy) @@ -6513,11 +6513,14 @@ life-range analysis. This option is eff @item -fno-branch-count-reg @opindex fno-branch-count-reg -Do not use ``decrement and branch'' instructions on a count register, -but instead generate a sequence of instructions that decrement a -register, compare it against zero, then branch based upon the result. -This option is only meaningful on architectures that support such -instructions, which include x86, PowerPC, IA-64 and S/390. +Avoid running a pass scanning for opportunities to use ``decrement and +branch'' instructions on a count register instead of generating sequences +of instructions that decrement a register, compare it against zero, and +then branch based upon the result. This option is only meaningful on +architectures that support such instructions, which include x86, PowerPC, +IA-64 and S/390. Note that the @option{-fno-branch-count-reg} option +doesn't remove the decrement and branch instructions from the generated +instruction stream introduced by other optimization passes. Enabled by default at @option{-O1} and higher.