[PATCH] D152914: [Draft] Make __builtin_cpu builtins target-independent

[Matt Arsenault via Phabricator via cfe-commits]

arsenm added inline comments.


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Comment at: llvm/include/llvm/IR/Intrinsics.td:903-907
+// Load of a value provided by the system library at a fixed address. Used for
+// accessing things like HWCAP word provided by GLIBC.
+def int_fixed_addr_ld
+    : DefaultAttrsIntrinsic<[llvm_i32_ty], [llvm_i32_ty],
+                            [IntrInaccessibleMemOnly, ImmArg<ArgIndex<0>>]>;
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nemanjai wrote:
> arsenm wrote:
> > From this description I don't understand what this is supposed to do. What does the input mean? Why does this use an i32 immarg and not a pointer? Why is the result only i32?
> That is fair enough. The description is fairly vague. I can try to improve it as per below. The parameter for this is not a pointer (i.e. not an address). It is an immediate that represents the index into the enumeration of values that are provided at a fixed address. The back end is then free to produce the actual fixed address and the load itself.
> The choice for the result type was admittedly arbitrary - on PPC, the values provided by GLIBC are 32-bit words.
> 
> Proposed comment describing this intrinsic:
> ```
> // This intrinsic is provided to allow back ends to emit load
> // instructions that load a value from a fixed address. The
> // parameter to the intrinsic is not an address, but an
> // immediate index into an enumeration that contains the
> // union of all such values available on all back ends.
> // An example is the HWCAP/HWCAP2/CPUID words
> // provided by GLIBC on PowerPC to allow fast access
> // to commonly used parts of AUXV. These are provided
> // at a fixed offset into the TCB (accessible through the
> // thread pointer).
> ```
This is baking an a very target specific implementation of device identification. Could you redefine this as something more abstract? Like returns a device ID integer, or a bool that some int input is supported?


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https://reviews.llvm.org/D152914