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 ----------------------------- 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
