[llvm] r182023 - PPC32 cannot form counter loops around i64 FP conversions

[Chris Lattner]

On May 16, 2013, at 9:52 AM, Hal Finkel <hfinkel at anl.gov> wrote:

> Author: hfinkel
> Date: Thu May 16 11:52:41 2013
> New Revision: 182023
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=182023&view=rev
> Log:
> PPC32 cannot form counter loops around i64 FP conversions
> 
> On PPC32, i64 FP conversions are implemented using runtime calls (which clobber
> the counter register). These must be excluded.

As you're finding, this is a really really fragile way to do this sort of thing.  You're building all kinds of assumptions of how lowering will happen into the pass in a very non-obvious way. I fear that this will be a continuous source of bugs as the compiler evolves.

Is it really that impractical to do counted loop formation after isel?

-Chris

> 
> Added:
>    llvm/trunk/test/CodeGen/PowerPC/ctrloop-fp64.ll
> Modified:
>    llvm/trunk/lib/Target/PowerPC/PPCCTRLoops.cpp
> 
> Modified: llvm/trunk/lib/Target/PowerPC/PPCCTRLoops.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCCTRLoops.cpp?rev=182023&r1=182022&r2=182023&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/PowerPC/PPCCTRLoops.cpp (original)
> +++ llvm/trunk/lib/Target/PowerPC/PPCCTRLoops.cpp Thu May 16 11:52:41 2013
> @@ -305,7 +305,11 @@ bool PPCCTRLoops::convertToCTRLoop(Loop
>                  isa<FPToUIInst>(J) || isa<FPToSIInst>(J)) {
>         CastInst *CI = cast<CastInst>(J);
>         if (CI->getSrcTy()->getScalarType()->isPPC_FP128Ty() ||
> -            CI->getDestTy()->getScalarType()->isPPC_FP128Ty())
> +            CI->getDestTy()->getScalarType()->isPPC_FP128Ty() ||
> +            (TT.isArch32Bit() &&
> +             (CI->getSrcTy()->getScalarType()->isIntegerTy(64) ||
> +              CI->getDestTy()->getScalarType()->isIntegerTy(64))
> +            ))
>           return MadeChange;
>       } else if (isa<IndirectBrInst>(J) || isa<InvokeInst>(J)) {
>         // On PowerPC, indirect jumps use the counter register.
> 
> Added: llvm/trunk/test/CodeGen/PowerPC/ctrloop-fp64.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/ctrloop-fp64.ll?rev=182023&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/PowerPC/ctrloop-fp64.ll (added)
> +++ llvm/trunk/test/CodeGen/PowerPC/ctrloop-fp64.ll Thu May 16 11:52:41 2013
> @@ -0,0 +1,28 @@
> +; RUN: llc < %s -mcpu=generic | FileCheck %s
> +
> +target datalayout = "E-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v128:128:128-n32"
> +target triple = "powerpc-unknown-linux-gnu"
> +
> +define i64 @foo(double* nocapture %n) nounwind readonly {
> +entry:
> +  br label %for.body
> +
> +for.body:                                         ; preds = %for.body, %entry
> +  %i.06 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
> +  %x.05 = phi i64 [ 0, %entry ], [ %conv1, %for.body ]
> +  %arrayidx = getelementptr inbounds double* %n, i32 %i.06
> +  %0 = load double* %arrayidx, align 8
> +  %conv = sitofp i64 %x.05 to double
> +  %add = fadd double %conv, %0
> +  %conv1 = fptosi double %add to i64
> +  %inc = add nsw i32 %i.06, 1
> +  %exitcond = icmp eq i32 %inc, 2048
> +  br i1 %exitcond, label %for.end, label %for.body
> +
> +for.end:                                          ; preds = %for.body
> +  ret i64 %conv1
> +}
> +
> +; CHECK: @foo
> +; CHECK-NOT: mtctr
> +
> 
> 
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