[llvm] r246937 - [PowerPC] Don't commute trivial rlwimi instructions

[Hal Finkel via llvm-commits]

Author: hfinkel
Date: Sat Sep  5 23:17:30 2015
New Revision: 246937

URL: http://llvm.org/viewvc/llvm-project?rev=246937&view=rev
Log:
[PowerPC] Don't commute trivial rlwimi instructions

To commute a trivial rlwimi instructions (meaning one with a full mask and zero
shift), we'd need to ability to form an all-zero mask (instead of an all-one
mask) using rlwimi. We can't represent this, however, and we'll miscompile code
if we try.

The code quality problem that this highlights (that SDAG simplification can
lead to us generating an ISD::OR node with a constant zero LHS) will be fixed
as a follow-up.

Fixes PR24719.

Added:
    llvm/trunk/test/CodeGen/PowerPC/no-rlwimi-trivial-commute.mir
Modified:
    llvm/trunk/lib/Target/PowerPC/PPCInstrInfo.cpp

Modified: llvm/trunk/lib/Target/PowerPC/PPCInstrInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/PowerPC/PPCInstrInfo.cpp?rev=246937&r1=246936&r2=246937&view=diff
==============================================================================
--- llvm/trunk/lib/Target/PowerPC/PPCInstrInfo.cpp (original)
+++ llvm/trunk/lib/Target/PowerPC/PPCInstrInfo.cpp Sat Sep  5 23:17:30 2015
@@ -571,6 +571,11 @@ PPCInstrInfo::commuteInstruction(Machine
   unsigned MB = MI->getOperand(4).getImm();
   unsigned ME = MI->getOperand(5).getImm();
 
+  // We can't commute a trivial mask (there is no way to represent an all-zero
+  // mask).
+  if (MB == 0 && ME == 31)
+    return nullptr;
+
   if (NewMI) {
     // Create a new instruction.
     unsigned Reg0 = ChangeReg0 ? Reg2 : MI->getOperand(0).getReg();

Added: llvm/trunk/test/CodeGen/PowerPC/no-rlwimi-trivial-commute.mir
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/PowerPC/no-rlwimi-trivial-commute.mir?rev=246937&view=auto
==============================================================================
--- llvm/trunk/test/CodeGen/PowerPC/no-rlwimi-trivial-commute.mir (added)
+++ llvm/trunk/test/CodeGen/PowerPC/no-rlwimi-trivial-commute.mir Sat Sep  5 23:17:30 2015
@@ -0,0 +1,92 @@
+# RUN: llc -start-after=dead-mi-elimination -stop-after=twoaddressinstruction -o /dev/null %s | FileCheck %s
+
+--- |
+  target datalayout = "E-m:e-i64:64-n32:64"
+  target triple = "powerpc64-unknown-linux-gnu"
+  
+  @d = global i32 15, align 4
+  @b = global i32* @d, align 8
+  @a = common global i32 0, align 4
+  
+  ; Function Attrs: nounwind
+  define signext i32 @main() #0 {
+  entry:
+    %0 = load i32*, i32** @b, align 8
+    %1 = load i32, i32* @a, align 4
+    %lnot = icmp eq i32 %1, 0
+    %lnot.ext = zext i1 %lnot to i32
+    %shr.i = lshr i32 2072, %lnot.ext
+    %call.lobit = lshr i32 %shr.i, 7
+    %2 = and i32 %call.lobit, 1
+    %3 = load i32, i32* %0, align 4
+    %or = or i32 %2, %3
+    store i32 %or, i32* %0, align 4
+    %4 = load i32, i32* @a, align 4
+    %lnot.1 = icmp eq i32 %4, 0
+    %lnot.ext.1 = zext i1 %lnot.1 to i32
+    %shr.i.1 = lshr i32 2072, %lnot.ext.1
+    %call.lobit.1 = lshr i32 %shr.i.1, 7
+    %5 = and i32 %call.lobit.1, 1
+    %or.1 = or i32 %5, %or
+    store i32 %or.1, i32* %0, align 4
+    ret i32 %or.1
+  }
+  
+  attributes #0 = { nounwind "target-cpu"="ppc64" }
+
+...
+---
+name:            main
+alignment:       2
+exposesReturnsTwice: false
+hasInlineAsm:    false
+isSSA:           true
+tracksRegLiveness: true
+tracksSubRegLiveness: false
+registers:       
+  - { id: 0, class: g8rc_and_g8rc_nox0 }
+  - { id: 1, class: g8rc_and_g8rc_nox0 }
+  - { id: 2, class: gprc }
+  - { id: 3, class: gprc }
+  - { id: 4, class: gprc }
+  - { id: 5, class: g8rc_and_g8rc_nox0 }
+  - { id: 6, class: g8rc_and_g8rc_nox0 }
+  - { id: 7, class: gprc }
+  - { id: 8, class: gprc }
+  - { id: 9, class: gprc }
+  - { id: 10, class: g8rc }
+frameInfo:       
+  isFrameAddressTaken: false
+  isReturnAddressTaken: false
+  hasStackMap:     false
+  hasPatchPoint:   false
+  stackSize:       0
+  offsetAdjustment: 0
+  maxAlignment:    0
+  adjustsStack:    false
+  hasCalls:        false
+  maxCallFrameSize: 0
+  hasOpaqueSPAdjustment: false
+  hasVAStart:      false
+  hasMustTailInVarArgFunc: false
+body:             |
+  bb.0.entry:
+    liveins: %x2
+
+    %0 = ADDIStocHA %x2, @b
+    %1 = LD target-flags(ppc-toc-lo) @b, killed %0 :: (load 8 from @b)
+    %2 = LWZ 0, %1 :: (load 4 from %ir.0)
+    %3 = LI 0
+    %4 = RLWIMI %3, killed %2, 0, 0, 31
+    ; CHECK-LABEL: name: main
+    ; CHECK: %[[REG1:[0-9]+]] = LI 0
+    ; CHECK: %[[REG2:[0-9]+]] = COPY %[[REG1]]
+    ; CHECK: %[[REG2]] = RLWIMI %[[REG2]], killed %2, 0, 0, 31
+    %8 = RLWIMI %3, %4, 0, 0, 31
+    STW %4, 0, %1 :: (store 4 into %ir.0)
+    %10 = EXTSW_32_64 %8
+    STW %8, 0, %1 :: (store 4 into %ir.0)
+    %x3 = COPY %10
+    BLR8 implicit %x3, implicit %lr8, implicit %rm
+
+...