Mispredict Feedback Example

This project demonstrates how to leverage PMU feedback with SEP on Windows or perf on Linux.

CMake Module

For portability, this project is described as a CMake project.

Included is an attempt at an HWPGO.cmake module which can help to add HWPGO feedback to an existing project.

The top-level CMakeLists.txt contains comments describing how to use HWPGO.cmake, which should be suitable for use in other CMake projects, although it has not been widely tested.

Usage

Building without HWPGO:

cmake -G Ninja -B nopgo -S . -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=icx
cmake --build nopgo --parallel

Building with HWPGO:

cmake -G Ninja -B hwpgo -S . -DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=icx -DHWPGO=On
cmake --build hwpgo --parallel

Results

Mispredicts Triggering Aggresive Speculation and cmov

In the case of the unpredictable executable the "if" condition in the hot loop is difficult to predict:

hwpgo-mispredict-example/unpredictable.c

Lines 24 to 31 in 493f07b

	if(s1[i] > 8000) {
	p = &s2[i] ;
	int z = i * i * i * i * i * i * i;
	nop(p, z);
	} else {
	p = &s3[i];
	nop(p, 3);
	}

Profitability heuristics based on static analysis are unlikely to eliminate control flow and implement the selection of p with cmov because it would require unconditionally computing z in case its value is needed. HWPGO allows the compiler to understand that more aggressive speculation is worthwhile, and so it implements the loop body with conditional moves rather than a branch.

$ time ./nopgo/unpredictable
./nopgo/unpredictable  0.99s user 0.00s system 99% cpu 0.992 total

$ time ./hwpgo/unpredictable
./hwpgo/3  0.19s user 0.00s system 99% cpu 0.197 total

The result is approximately a 5x speedup.

Lack of Mispredicts Preserving Control Flow

The predictable example shows the case where a branch is predictable and cmov conversion would be harmful.

Although the branch remains taken about 50% of the time, the taken/not-taken pattern is predictable to hardware. HWPGO generates an empty mispredict profile in this case, indicating that there is no mispredict problem. As a result, cmov is not used and performance is preserved.

hwpgo-mispredict-example/predictable.c

Lines 22 to 29 in 493f07b

	if(s1[i] % 2) {
	p = &s2[i] ;
	int z = i * i * i * i * i * i * i;
	nop(p, z);
	} else {
	p = &s3[i];
	nop(p, 3);
	}