Uros, On Mon, May 20, 2019 at 3:29 AM Uros Bizjak <ubiz...@gmail.com> wrote: > The CPUID documentation from Intel Architectures Developer's manual is > crystal clear on the implementation: > > --quote-- > CPUID—CPU Identification > > Returns processor identification and feature > information to the EAX, EBX, ECX, and EDX > registers, as determined by input entered in > EAX (in some cases, ECX as well). > --quote--
That is for the current versions of the documentation. Earlier versions are different (see below). > So, I dont see why we should implement the workaround for an > implementation bug in a widely used header for a relatively obscure > target. Please also note that there are other places in the compiler > that assume that the instruction returns values as specified above, > not to mention other applications that assumes the same. > > I think proposed target specific bugfix should be implemented > elsewhere, not in the cpuid.h header that assumes the specified > functionality. I did some research and found the Intel CPUID document that existed when the WinChip was manufactured (dated December 1996): http://bitsavers.trailing-edge.com/components/intel/appNotes/AP-485_Intel_Processor_Identification_and_the_CPUID_Instruction_Dec96.pdf It is ALSO silent about the contents of %ebx and %ecx after a cpuid() with %eax = 1. The Winchip met the Intel _published_ standard that existed at the time. Intel changed the standard a long time afterwards. This means that all the following processor manufacturers (and example cpus) of that era might demonstrate this behavior: AMD (Am486 DX2/DX4, Am5x86, K5) Centaur (WinChip C6, Winchip 2) NexGen (Nx586) Cyrix (Cx486DX2/DX4, 5x86, 6x86, 6x86MX) That is a lot of processors. Admittedly they are old, but they are still officially supported by GNU/Linux. Since the patch only introduces four (4) more opcodes, it is not computationally expensive. Here is the diff: test $0x200000,%eax je 0xb7e214c0 <rdrand+128> - xor %eax,%eax + xor %edi,%edi + mov %edi,%eax cpuid test %eax,%eax je 0xb7e214c0 <rdrand+128> + mov %edi,%ebx + mov %edi,%ecx + mov %edi,%edx mov $0x1,%eax cpuid Lastly, this is just an example of "initialized your variables before you use them". - Matthew
