On Tue, Jul 2, 2024 at 4:54 AM liuhongt <hongtao....@intel.com> wrote: > > From: "H.J. Lu" <hjl.to...@gmail.com> > > According to Intel® 64 and IA-32 Architectures Optimization Reference > Manual[1], Branch Hint is updated for Redwood Cove. > > --------cut from [1]------------------------- > Starting with the Redwood Cove microarchitecture, if the predictor has > no stored information about a branch, the branch has the Intel® SSE2 > branch taken hint (i.e., instruction prefix 3EH), When the codec > decodes the branch, it flips the branch’s prediction from not-taken to > taken. It then flushes the pipeline in front of it and steers this > pipeline to fetch the taken path of the branch. > --------cut end -----------------------------
The above reads like it would be worth splitting branc_prediction_hits into branch_prediction_hints_taken and branch_prediction_hints_not_taken given not-taken is the default and thus will just increase code size? > For -mtune-ctrl=branch_prediction_hints, always generate branch hint for > conditional branches, this tune is disabled by default. > > [1] > https://www.intel.com/content/www/us/en/content-details/821612/intel-64-and-ia-32-architectures-optimization-reference-manual-volume-1.html > > Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}. > Ready push to trunk. > > gcc/ > > * config/i386/i386.cc (ix86_print_operand): Always generate > branch hint for conditional branches. > --- > gcc/config/i386/i386.cc | 24 +++++------------------- > 1 file changed, 5 insertions(+), 19 deletions(-) > > diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc > index 1f71ed04be6..9992b9d6186 100644 > --- a/gcc/config/i386/i386.cc > +++ b/gcc/config/i386/i386.cc > @@ -14050,25 +14050,11 @@ ix86_print_operand (FILE *file, rtx x, int code) > int pred_val = profile_probability::from_reg_br_prob_note > (XINT (x, 0)).to_reg_br_prob_base (); > > - if (pred_val < REG_BR_PROB_BASE * 45 / 100 > - || pred_val > REG_BR_PROB_BASE * 55 / 100) > - { > - bool taken = pred_val > REG_BR_PROB_BASE / 2; > - bool cputaken > - = final_forward_branch_p (current_output_insn) == 0; > - > - /* Emit hints only in the case default branch prediction > - heuristics would fail. */ > - if (taken != cputaken) > - { > - /* We use 3e (DS) prefix for taken branches and > - 2e (CS) prefix for not taken branches. */ > - if (taken) > - fputs ("ds ; ", file); > - else > - fputs ("cs ; ", file); > - } > - } > + bool taken = pred_val > REG_BR_PROB_BASE / 2; > + /* We use 3e (DS) prefix for taken branches and > + 2e (CS) prefix for not taken branches. */ > + if (taken) > + fputs ("ds ; ", file); > } > return; > } > -- > 2.31.1 >
