Apparatus and method for speculatively updating global history and restoring same on branch misprediction detection

A method for performing branch prediction in a microprocessor, comprising:

• predicting an outcome of a branch instruction based on a global history of previous branch instruction outcomes of a plurality of branch instructions stored in a first storage element;
• updating said global history in said first storage element in response to said predicting said outcome of said branch instruction prior to resolution of said outcome of said branch instruction;
• predicting an outcome of a second branch instruction based on said global history stored in said first storage element after said updating said global history in said first storage element in response to said predicting said outcome of said first branch instruction and prior to resolution of said outcome of said first branch instruction;
• copying said global history in said first storage element to a second storage element prior to said updating said global history in said first storage element in response to said predicting said outcome of said first branch instruction;
• determining said outcome of said first branch instruction;
• and
• comparing said outcome of said first branch instruction and said prediction of said outcome of said first branch instruction to determine if said prediction of said outcome of said first branch instruction is correct;
• and
• copying said global history in said second storage element to said first storage element and updating said global history in said first storage element in response to said outcome of said first branch instruction if said prediction of said outcome of said first branch instruction is incorrect.

A microprocessor capable of performing branch prediction, comprising:

• instruction fetch logic configured to fetch a branch instruction;
• execution logic coupled to said instruction fetch logic configured to resolve an outcome of said branch instruction;
• and
• a branch predictor coupled to said instruction fetch logic and said execution logic, comprising:
• a first storage element configured to store a global history of previous branch instruction outcomes of a plurality of branch instructions;
• branch control coupled to said first storage element comprising a prediction output for making a prediction of an outcome of said branch instruction based on said global history stored in said first storage element, wherein said instruction fetch logic is configured to fetch a next instruction in response to said prediction;
• wherein said branch control is configured to update said global history in said first storage element in response to said prediction prior to resolution of said outcome of said branch instruction by said execution logic;
• wherein said branch control is further configured to make a prediction of an outcome of a second branch instruction fetched by said instruction fetch logic based on said global history stored in said first storage element after said global history in said first storage element has been updated in response to said prediction of said outcome of said first branch instruction and prior to resolution of said outcome of said first branch instruction by said execution logic;
• a second storage element coupled to said branch control and to said first storage element;
• wherein said branch control is further configured to copy said global history in said first storage element to said second storage element prior to updating said global history in said first storage element in response to said prediction of said outcome of said first branch instruction;
• a result input configured to receive from said execution logic an indication of said outcome of said first branch instruction;
• and
• a comparator configured to compare said outcome of said first branch instruction and said prediction of said outcome of said first branch instruction to determine if said prediction of said outcome of said first branch instruction is correct and if said prediction of said outcome of said first branch instruction is incorrect to copy said global history in said second storage element to said first storage element and to update said global history in said first storage element in response to said outcome of said first branch instruction.