aaron.ballman added inline comments. ================ Comment at: include/clang/CodeGen/CGFunctionInfo.h:479 @@ +478,3 @@ + /// Whether this function saved caller registers. + unsigned NoCallerSavedRegs : 1; + ---------------- erichkeane wrote: > aaron.ballman wrote: > > This is unfortunate as it will bump the bit-field length to 33 bits, which > > seems rather wasteful. Are there any bits we can steal to bring this back > > down to a 32-bit bit-field? > I implemented this additional patch, but don't really know a TON about this > area, so I have a few ideas but would like to get direction on it if > possible. I had the following ideas of where to get a few more bits: > > CallingConvention/EffectiveCallingConvention/ASTCallingConvention: This > structure stores a pre-converted calling convention to the llvm::CallingConv > enum (llvm/include/llvm/IR/CallingConv.h). I'll note that the legal values > for this go up to 1023 (so 8 bits isn't enough anyway!), though only up to 91 > are currently used. > > HOWEVER, the clang CallingConv (include/clang/Basic/Specifiers.h :233) only > has 16 items in it. If we were instead to store THAT and convert upon access > (a simple switch statement, already used constructing this value, see > ClangCallConvToLLVMCallConv), we could get away with 6 or 7 bits each, saving > this 3-6 bits total, yet have way more than enough room for expansion. > > HasRegParm: This field might be possible to eliminate. According to the GCC > RegParm docs (I don't see a clang one?) here > (https://gcc.gnu.org/onlinedocs/gcc/x86-Function-Attributes.html), ", the > regparm attribute causes the compiler to pass arguments number one to number > if they are of integral type in registers EAX, EDX, and ECX instead of on the > stack". > > It seems that 0 for "RegParm" should be illegal, so I wonder if we can treat > "RegParm==0" as "HasRegParm==false" and eliminate storing that. > > In my opinion, the 1st one gives us way more room for the future and corrects > a possible future bug. The 2nd is likely a lower touch, though it could > possibly change behavior (see discussion here > https://www.coreboot.org/pipermail/coreboot/2008-October/040406.html) as > regparm(0) is seemingly accepted by both compilers. I DO note that this > comment notes that 'regparm 0' is the default behavior, so I'm not sure what > change that would do. > > Either way, I suspect this change should be a separate commit (though I would > figure making it a pre-req for this patch would be the right way to go). If > you give some guidance as to which you think would be better, I can put a > patch together. > > -Erich > I think that `unsigned ASTCallingConvention : 8;` can be safely reduced. This tracks a `clang::CallingConv` value, the maximum of which is 15. So I think the way to do this is to reduce ASTCallingConvention from 8 to 7 bits and then pack yours in as well.
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