[llvm] r283086 - [PowerPC] Account for the ELFv2 function prologue during branch selection

[Hal Finkel via llvm-commits]

----- Original Message -----
> From: "Eric Christopher" <echristo at gmail.com>
> To: "Hal Finkel" <hfinkel at anl.gov>, llvm-commits at lists.llvm.org
> Sent: Thursday, October 20, 2016 8:46:08 PM
> Subject: Re: [llvm] r283086 - [PowerPC] Account for the ELFv2 function prologue during branch selection
> 
> 
> Hrm. Would it make sense to have the code in EmitFunctionBodyStart
> emit instructions into the first basic block instead? (Will that
> work here?)
> 

The instructions would need to be added before branch relaxation runs. We could do this with a separate pass that runs before branch relaxation. Not sure it is worth the complexity. Perhaps the next bug that comes up because of this I'll change my mind? ;)

 -Hal

> 
> Just thinking out loud.
> 
> 
> -eric
> 
> 
> On Sun, Oct 2, 2016 at 9:15 PM Hal Finkel via llvm-commits <
> llvm-commits at lists.llvm.org > wrote:
> 
> 
> Author: hfinkel
> Date: Sun Oct 2 23:06:44 2016
> New Revision: 283086
> 
> URL: http://llvm.org/viewvc/llvm-project?rev=283086&view=rev
> Log:
> [PowerPC] Account for the ELFv2 function prologue during branch
> selection
> 
> The PPC branch-selection pass, which performs branch relaxation,
> needs to
> account for the padding that might be introduced to satisfy block
> alignment
> requirements. We were assuming that the first block was at offset
> zero (i.e.
> had the alignment of the function itself), but under the ELFv2 ABI, a
> global
> entry function prologue is added to the first block, and it is a
> two-instruction sequence (i.e. eight-bytes long). If the function has
> 16-byte
> alignment, the fact that the first block is eight bytes offset from
> the start
> of the function is relevant to calculating where padding will be
> added in
> between later blocks.
> 
> Unfortunately, I don't have a small test case.
> 
> Modified:
> llvm/trunk/lib/Target/PowerPC/PPCAsmPrinter.cpp
> llvm/trunk/lib/Target/PowerPC/PPCBranchSelector.cpp
> 
> Modified: llvm/trunk/lib/Target/PowerPC/PPCAsmPrinter.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCAsmPrinter.cpp?rev=283086&r1=283085&r2=283086&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/PowerPC/PPCAsmPrinter.cpp (original)
> +++ llvm/trunk/lib/Target/PowerPC/PPCAsmPrinter.cpp Sun Oct 2
> 23:06:44 2016
> @@ -1199,6 +1199,9 @@ void PPCLinuxAsmPrinter::EmitFunctionBod
> if (Subtarget->isELFv2ABI()
> // Only do all that if the function uses r2 in the first place.
> && !MF->getRegInfo().use_empty(PPC::X2)) {
> + // Note: The logic here must be synchronized with the code in the
> + // branch-selection pass which sets the offset of the first block
> in the
> + // function. This matters because it affects the alignment.
> const PPCFunctionInfo *PPCFI = MF->getInfo<PPCFunctionInfo>();
> 
> MCSymbol *GlobalEntryLabel = PPCFI->getGlobalEPSymbol();
> 
> Modified: llvm/trunk/lib/Target/PowerPC/PPCBranchSelector.cpp
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCBranchSelector.cpp?rev=283086&r1=283085&r2=283086&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/PowerPC/PPCBranchSelector.cpp (original)
> +++ llvm/trunk/lib/Target/PowerPC/PPCBranchSelector.cpp Sun Oct 2
> 23:06:44 2016
> @@ -19,8 +19,10 @@
> #include "MCTargetDesc/PPCPredicates.h"
> #include "PPCInstrBuilder.h"
> #include "PPCInstrInfo.h"
> +#include "PPCSubtarget.h"
> #include "llvm/ADT/Statistic.h"
> #include "llvm/CodeGen/MachineFunctionPass.h"
> +#include "llvm/CodeGen/MachineRegisterInfo.h"
> #include "llvm/Support/MathExtras.h"
> #include "llvm/Target/TargetMachine.h"
> #include "llvm/Target/TargetSubtargetInfo.h"
> @@ -92,8 +94,19 @@ bool PPCBSel::runOnMachineFunction(Machi
> return AlignAmt + OffsetToAlignment(Offset, AlignAmt);
> };
> 
> + // We need to be careful about the offset of the first block in the
> function
> + // because it might not have the function's alignment. This happens
> because,
> + // under the ELFv2 ABI, for functions which require a TOC pointer,
> we add a
> + // two-instruction sequence to the start of the function.
> + // Note: This needs to be synchronized with the check in
> + // PPCLinuxAsmPrinter::EmitFunctionBodyStart.
> + unsigned InitialOffset = 0;
> + if (Fn.getSubtarget<PPCSubtarget>().isELFv2ABI() &&
> + !Fn.getRegInfo().use_empty(PPC::X2))
> + InitialOffset = 8;
> +
> // Measure each MBB and compute a size for the entire function.
> - unsigned FuncSize = 0;
> + unsigned FuncSize = InitialOffset;
> for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI !=
> E;
> ++MFI) {
> MachineBasicBlock *MBB = &*MFI;
> @@ -240,7 +253,7 @@ bool PPCBSel::runOnMachineFunction(Machi
> if (MadeChange) {
> // If we're going to iterate again, make sure we've updated our
> // padding-based contributions to the block sizes.
> - unsigned Offset = 0;
> + unsigned Offset = InitialOffset;
> for (MachineFunction::iterator MFI = Fn.begin(), E = Fn.end(); MFI !=
> E;
> ++MFI) {
> MachineBasicBlock *MBB = &*MFI;
> 
> 
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory