[PATCH] D114116: [clang][ARM] relax -mtp=cp15 for ARMv6 non-thumb cases

[Peter Smith via Phabricator via cfe-commits]

peter.smith added a comment.

Apologies had to go on a bit of a dive through the documentation. I've put some comments inline and some links to other documents that may help.



================
Comment at: clang/lib/Driver/ToolChains/Arch/ARM.cpp:150
 
+// The backend does not have support for hard thread pointers when targeting
+// Thumb1.
----------------
Will be worth saying `CP15 C13 ThreadID` somewhere to make it easier to look up in the documentation.


================
Comment at: clang/lib/Driver/ToolChains/Arch/ARM.cpp:155
+  llvm::ARM::ArchKind AK = llvm::ARM::parseArch(Triple.getArchName());
+  return Ver >= 7 || AK == llvm::ARM::ArchKind::ARMV6T2 ||
+         (Ver == 6 && Triple.isARM());
----------------
Are we restricting based on whether the threadid register is present or whether the instructions are available to access the cp15 registers?

If we're going by whether the CPU has the register present then it will be
* A and R profile (not M profile, even the ones that have Thumb2)
* V6K (includes ARM1176 but not ARM1156t2-s which has Thumb-2!) and V7+ (A and R profile)

If we're going by the instructions to write to CP15 then it is:
* Arm state (everything)
* Thumb2 (v7 + v6t2)

The above seems to be a blend of the two. Is it worth choosing one form or the other? GCC seems to use the latter. I guess using this option falls into the I know what I'm doing area that accessing named system registers comes into. If the kernel supports it the stricter version may help catch more mistakes though.

The v7 A/R Arm ARM https://developer.arm.com/documentation/ddi0403/ed
has in `D12.7.21 CP15 c13, Context ID support`
``` An ARMv6K implementation requires the Software Thread ID registers described in VMSA CP15 c13
register summary, Process, context and thread ID registers on page B3-1474. ```

The Arm 1156-s (the only v6t2 processor) TRM https://developer.arm.com/documentation/ddi0338/g/system-control-coprocessor/system-control-processor-registers/c13--process-id-register?lang=en which shows only one process ID register under opcode 1 accessed via:
```
MRC p15, 0, <Rd>, c13, c0, 1 ;Read Process ID Register
```
Whereas the ThreadID register is opcode 3 on CPUs that are v6k and v7.


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