[PATCH] D39173: [MachineCSE] Remove redudant TLS access instructions.

[Tony Jiang via Phabricator via llvm-commits]

jtony created this revision.

The instructions addis,addi, bl are used to calculate the address of TLS thread local variables. These TLS access code sequences are generated repeatedly every time the thread local variable is accessed. By communicating to Machine CSE that X2 is guaranteed to have the same value within the same function call (so called Caller Preserved Physical Register), the redundant TLS access code sequences are  cleaned up.

Note: for the test case included in the patch, before the fix the same TLS access code sequences (`addis/addi/bl`) is generated twice. After the fix, it is only generated once. The test case tests the new behaviour after the fix.


https://reviews.llvm.org/D39173

Files:
  lib/CodeGen/MachineCSE.cpp
  test/CodeGen/PowerPC/MCSE-caller-preserved-reg.ll


Index: test/CodeGen/PowerPC/MCSE-caller-preserved-reg.ll
===================================================================
--- /dev/null
+++ test/CodeGen/PowerPC/MCSE-caller-preserved-reg.ll
@@ -0,0 +1,66 @@
+; RUN: llc -verify-machineinstrs -mtriple=powerpc64le-unknown-linux-gnu < %s | FileCheck %s
+; The instructions addis,addi, bl are used to calculate the address of TLS
+; thread local variables. These TLS access code sequences are generated
+; repeatedly every time the thread local variable is accessed. By communicating
+; to Machine CSE that X2 is guaranteed to have the same value within the same
+; function call (so called Caller Preserved Physical Register), the redudant
+; TLS access code sequences are cleaned up.
+
+%"struct.CC::TT" = type { i64, i32 }
+%class.CC = type { %struct.SS }
+%struct.SS = type { void ()* }
+
+ at _ZN2CC2ccE = external thread_local global %"struct.CC::TT", align 8
+
+define noalias i8* @_ZN2CC3funEv(%class.CC* %this) {
+; CHECK-LABEL: _ZN2CC3funEv:
+; CHECK:    mflr 0
+; CHECK-NEXT:    std 0, 16(1)
+; CHECK-NEXT:    stdu 1, -48(1)
+; CHECK-NEXT:  .Lcfi0:
+; CHECK-NEXT:    .cfi_def_cfa_offset 48
+; CHECK-NEXT:  .Lcfi1:
+; CHECK-NEXT:    .cfi_offset lr, 16
+; CHECK-NEXT:  .Lcfi2:
+; CHECK-NEXT:    .cfi_offset r30, -16
+; CHECK-NEXT:    std 30, 32(1)
+; CHECK-NEXT:    mr 30, 3
+; CHECK-NEXT:    ld 12, 0(30)
+; CHECK-NEXT:    std 2, 24(1)
+; CHECK-NEXT:    mtctr 12
+; CHECK-NEXT:    bctrl
+; CHECK-NEXT:    ld 2, 24(1)
+; CHECK-NEXT:    addis 3, 2, _ZN2CC2ccE at got@tlsgd at ha
+; CHECK-NEXT:    addi 3, 3, _ZN2CC2ccE at got@tlsgd at l
+; CHECK-NEXT:    bl __tls_get_addr(_ZN2CC2ccE at tlsgd)
+; CHECK-NEXT:    nop
+; CHECK-NEXT:    ld 4, 0(3)
+; CHECK-NEXT:    cmpldi 4, 0
+; CHECK-NEXT:    beq 0, .LBB0_2
+; CHECK:    addi 4, 3, 8
+; CHECK-NEXT:    mr 3, 30
+; CHECK-NEXT:    bl _ZN2CC3barEPi
+; CHECK-NEXT:    nop
+; CHECK:    ld 30, 32(1)
+; CHECK-NEXT:    li 3, 0
+; CHECK-NEXT:    addi 1, 1, 48
+; CHECK-NEXT:    ld 0, 16(1)
+; CHECK-NEXT:    mtlr 0
+; CHECK-NEXT:    blr
+entry:
+  %foo = getelementptr inbounds %class.CC, %class.CC* %this, i64 0, i32 0, i32 0
+  %0 = load void ()*, void ()** %foo, align 8
+  tail call void %0()
+  %1 = load i64, i64* getelementptr inbounds (%"struct.CC::TT", %"struct.CC::TT"* @_ZN2CC2ccE, i64 0, i32 0)
+  %tobool = icmp eq i64 %1, 0
+  br i1 %tobool, label %if.end, label %if.then
+
+if.then:
+  tail call void @_ZN2CC3barEPi(%class.CC* nonnull %this, i32* getelementptr inbounds (%"struct.CC::TT", %"struct.CC::TT"* @_ZN2CC2ccE, i64 0, i32 1))
+  br label %if.end
+
+if.end:
+  ret i8* null
+}
+
+declare void @_ZN2CC3barEPi(%class.CC*, i32*)
Index: lib/CodeGen/MachineCSE.cpp
===================================================================
--- lib/CodeGen/MachineCSE.cpp
+++ lib/CodeGen/MachineCSE.cpp
@@ -251,7 +251,8 @@
     if (TargetRegisterInfo::isVirtualRegister(Reg))
       continue;
     // Reading constant physregs is ok.
-    if (!MRI->isConstantPhysReg(Reg))
+    if (!MRI->isConstantPhysReg(Reg) &&
+        !(TRI->isCallerPreservedPhysReg(Reg, *MI->getParent()->getParent())))
       for (MCRegAliasIterator AI(Reg, TRI, true); AI.isValid(); ++AI)
         PhysRefs.insert(*AI);
   }


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