Starting from certain version of Visual Studio C compiler (I don’t
have the exact version. I am using VS2019), CpuDeadLoop is optimized
quite well by compiler.
The compiler does not generate instructions that jump out of the loop
when the "Index" is non-zero.
It becomes harder/impossible for developers to break out of the dead-loop
in debugger.
The new version of CpuDeadLoop() compares a volatile global to a volatile
local. This forces 2 reads and a comparison on every loop iteration.
The local variable can be set to 1 to exit the loop without modifying the
global variable.
Using VS2019 with max opt enabled, The dead-loop can be exit by setting
Index to 1 in a debugger.
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3737
Apply uncrustify changes to .c/.h files in the MdePkg package
Cc: Andrew Fish <afish@apple.com>
Cc: Leif Lindholm <leif@nuviainc.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
CpuPause() might allow the CPU to go into a lower power state
state while we spin.
On X86, CpuPause() executes a PAUSE instruction which the Intel
and AMD specs describe as follows:
Intel:
"PAUSE: An additional function of the PAUSE instruction is to reduce
the power consumed by a processor while executing a spin loop. A
processor can execute a spin-wait loop extremely quickly, causing the
processor to consume a lot of power while it waits for the resource it
is spinning on to become available. Inserting a pause instruction in a
spin-wait loop greatly reduces the processor?s power consumption."
AMD:
"PAUSE: Improves the performance of spin loops, by providing a hint to
the processor that the current code is in a spin loop. The processor
may use this to optimize power consumption while in the spin loop.
Architecturally, this instruction behaves like a NOP instruction."
On RISC-V and ARM64, CpuPause() executes a NOP, which is no worse than
the tight loop we have.
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>
Ray Ni
Michael Kubacki
Ankur Arora
Michael D Kinney
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