[PATCH] D42154: Don't generate inline atomics for i386/i486

[Craig Topper via Phabricator via cfe-commits]

craig.topper added inline comments.


================
Comment at: lib/Basic/Targets/X86.h:472
+    CPUKind Kind = getCPUKind(Opts.CPU);
+    if (Kind >= CK_i586 || Kind == CK_Generic)
+      MaxAtomicPromoteWidth = MaxAtomicInlineWidth = 64;
----------------
wmi wrote:
> wmi wrote:
> > craig.topper wrote:
> > > craig.topper wrote:
> > > > efriedma wrote:
> > > > > What exactly does "CK_Generic" mean here?  Under what circumstances does it show up? We should have a specific default for every supported target, if user doesn't specify a CPU with -march.
> > > > CK_Generic is the default for 32-bit mode with no -march. CK_x86_64 is the default for 64-bit mode with no march.
> > > > 
> > > > I'm not sure we can assume CK_Generic is 586 or better. We don't assume that in the preprocessor defines.
> > > > 
> > > > ```
> > > >   if (CPU >= CK_i486) {
> > > >     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1");
> > > >     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2");
> > > >     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4");
> > > >   }
> > > >   if (CPU >= CK_i586)
> > > >     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8");
> > > >   if (HasCX16)
> > > >     Builder.defineMacro("__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16");
> > > > ```
> > > Maybe we don't default to "generic" anywhere.  Its very platform specific. getX86TargetCPU in lib/Driver/ToolChains/Arch/X86.cpp contains the magic I think.
> > Like the command "clang -cc1 -triple=i686-linux-gnu", the cpu will be set to CK_Generic. I want to set the MaxAtomicInlineWidth of the default target to 64 (same as before) to minimize the churn. In the patch, MaxAtomicInlineWidth will only be changed if i386/i486 are explicitly specified.
> In X86TargetInfo::getCPUKind (tools/clang/lib/Basic/Targets/X86.cpp), we default to "generic", for both 32 bits and 64 bits modes.
You're right. Without the driver we don't have a CPU string to use and end up defaulting to CK_Generic.


Repository:
  rC Clang

https://reviews.llvm.org/D42154