[llvm] r267634 - [X86] Make sure it is safe to clobber EFLAGS, if need be, when choosing

[Hans Wennborg via llvm-commits]

+Tom: this might be a good candidate for 3.8.1.
+Nadav for X86 ownership

On Tue, Apr 26, 2016 at 4:44 PM, Quentin Colombet via llvm-commits
<llvm-commits at lists.llvm.org> wrote:
> Author: qcolombet
> Date: Tue Apr 26 18:44:14 2016
> New Revision: 267634
>
> URL: http://llvm.org/viewvc/llvm-project?rev=267634&view=rev
> Log:
> [X86] Make sure it is safe to clobber EFLAGS, if need be, when choosing
> the prologue.
>
> Do not use basic blocks that have EFLAGS live-in as prologue if we need
> to realign the stack. Realigning the stack uses AND instruction and this
> clobbers EFLAGS.
>
> An other alternative would have been to save and restore EFLAGS around
> the stack realignment code, but this is likely inefficient.
>
> Fixes PR27531.
>
> Added:
>     llvm/trunk/test/CodeGen/X86/i686-win-shrink-wrapping.ll
> Modified:
>     llvm/trunk/lib/Target/X86/X86FrameLowering.cpp
>     llvm/trunk/lib/Target/X86/X86FrameLowering.h
>
> Modified: llvm/trunk/lib/Target/X86/X86FrameLowering.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86FrameLowering.cpp?rev=267634&r1=267633&r2=267634&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/X86/X86FrameLowering.cpp (original)
> +++ llvm/trunk/lib/Target/X86/X86FrameLowering.cpp Tue Apr 26 18:44:14 2016
> @@ -2609,6 +2609,12 @@ eliminateCallFramePseudoInstr(MachineFun
>    return I;
>  }
>
> +bool X86FrameLowering::canUseAsPrologue(const MachineBasicBlock &MBB) const {
> +  assert(MBB.getParent() && "Block is not attached to a function!");
> +  const MachineFunction &MF = *MBB.getParent();
> +  return !TRI->needsStackRealignment(MF) || !MBB.isLiveIn(X86::EFLAGS);
> +}
> +
>  bool X86FrameLowering::canUseAsEpilogue(const MachineBasicBlock &MBB) const {
>    assert(MBB.getParent() && "Block is not attached to a function!");
>
>
> Modified: llvm/trunk/lib/Target/X86/X86FrameLowering.h
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86FrameLowering.h?rev=267634&r1=267633&r2=267634&view=diff
> ==============================================================================
> --- llvm/trunk/lib/Target/X86/X86FrameLowering.h (original)
> +++ llvm/trunk/lib/Target/X86/X86FrameLowering.h Tue Apr 26 18:44:14 2016
> @@ -127,6 +127,16 @@ public:
>    /// Check that LEA can be used on SP in an epilogue sequence for \p MF.
>    bool canUseLEAForSPInEpilogue(const MachineFunction &MF) const;
>
> +  /// Check whether or not the given \p MBB can be used as a prologue
> +  /// for the target.
> +  /// The prologue will be inserted first in this basic block.
> +  /// This method is used by the shrink-wrapping pass to decide if
> +  /// \p MBB will be correctly handled by the target.
> +  /// As soon as the target enable shrink-wrapping without overriding
> +  /// this method, we assume that each basic block is a valid
> +  /// prologue.
> +  bool canUseAsPrologue(const MachineBasicBlock &MBB) const override;
> +
>    /// Check whether or not the given \p MBB can be used as a epilogue
>    /// for the target.
>    /// The epilogue will be inserted before the first terminator of that block.
>
> Added: llvm/trunk/test/CodeGen/X86/i686-win-shrink-wrapping.ll
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/i686-win-shrink-wrapping.ll?rev=267634&view=auto
> ==============================================================================
> --- llvm/trunk/test/CodeGen/X86/i686-win-shrink-wrapping.ll (added)
> +++ llvm/trunk/test/CodeGen/X86/i686-win-shrink-wrapping.ll Tue Apr 26 18:44:14 2016
> @@ -0,0 +1,44 @@
> +; RUN: llc %s -o - -enable-shrink-wrap=true | FileCheck %s --check-prefix=CHECK --check-prefix=ENABLE
> +; RUN: llc %s -o - -enable-shrink-wrap=false | FileCheck %s --check-prefix=CHECK --check-prefix=DISABLE
> +target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
> +target triple = "i686-pc-windows-msvc18.0.0"
> +
> +%struct.S = type { i32 }
> +
> +; Check that we do not use a basic block that has EFLAGS as live-in
> +; if we need to realign the stack.
> +; PR27531.
> +; CHECK-LABEL: stackRealignment:
> +; Prologue code.
> +; CHECK: pushl
> +; Make sure we actually perform some stack realignment.
> +; CHECK: andl ${{[-0-9]+}}, %esp
> +; This is the end of the entry block.
> +; The prologue should have happened before that point because past
> +; this point, EFLAGS is live.
> +; CHECK: jg
> +define x86_thiscallcc void @stackRealignment(%struct.S* %this) {
> +entry:
> +  %data = alloca [1 x i32], align 4
> +  %d = alloca double, align 8
> +  %tmp = bitcast [1 x i32]* %data to i8*
> +  %arrayinit.begin = getelementptr inbounds [1 x i32], [1 x i32]* %data, i32 0, i32 0
> +  %x_ = getelementptr inbounds %struct.S, %struct.S* %this, i32 0, i32 0
> +  %tmp1 = load i32, i32* %x_, align 4
> +  %cmp = icmp sgt i32 %tmp1, 32
> +  %cond = select i1 %cmp, i32 42, i32 128
> +  store i32 %cond, i32* %arrayinit.begin, align 4
> +  %cmp3 = icmp slt i32 %tmp1, 32
> +  br i1 %cmp3, label %cleanup, label %if.end
> +
> +if.end:                                           ; preds = %entry
> +  %tmp2 = bitcast double* %d to i8*
> +  call x86_thiscallcc void @bar(%struct.S* nonnull %this, i32* %arrayinit.begin, double* nonnull %d)
> +  br label %cleanup
> +
> +cleanup:                                          ; preds = %if.end, %entry
> +  ret void
> +}
> +
> +; Function Attrs: optsize
> +declare x86_thiscallcc void @bar(%struct.S*, i32*, double*)