[PATCH] D74454: Use SETNE directly rather than SUB/SETNE 0 for stack guard check

[James Clarke via Phabricator via llvm-commits]

jrtc27 created this revision.
jrtc27 added reviewers: bogner, lebedev.ri, efriedma, t.p.northover, uweigand, sunfish.
Herald added subscribers: llvm-commits, aheejin, hiraditya, jgravelle-google, arichardson, sbc100, dschuff.
Herald added a project: LLVM.

Backends should fold the subtraction into the comparison, but not all
seem to. Moreover, on targets where pointers are not integers, such as
CHERI, an integer subtraction is not appropriate. Instead we should just
compare the two pointers directly, as this should work everywhere and
potentially generate more efficient code.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D74454

Files:
  llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
  llvm/test/CodeGen/SystemZ/stack-guard.ll
  llvm/test/CodeGen/Thumb/stack_guard_remat.ll
  llvm/test/CodeGen/WebAssembly/stack-protector.ll


Index: llvm/test/CodeGen/WebAssembly/stack-protector.ll
===================================================================
--- llvm/test/CodeGen/WebAssembly/stack-protector.ll
+++ llvm/test/CodeGen/WebAssembly/stack-protector.ll
@@ -1,7 +1,7 @@
 ; RUN: llc -verify-machineinstrs -mtriple=wasm32-unknown-unknown < %s | FileCheck -check-prefix=WASM32 %s
 
 ; WASM32: i32.load        28
-; WASM32-NEXT: i32.sub
+; WASM32-NEXT: i32.ne
 ; WASM32-NEXT: br_if           0
 
 ; WASM32: __stack_chk_fail
Index: llvm/test/CodeGen/Thumb/stack_guard_remat.ll
===================================================================
--- llvm/test/CodeGen/Thumb/stack_guard_remat.ll
+++ llvm/test/CodeGen/Thumb/stack_guard_remat.ll
@@ -11,7 +11,7 @@
 ;PIC-NEXT:   add [[ORIGINAL_GUARD]], pc
 ;PIC-NEXT:   ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
 ;PIC-NEXT:   ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
-;PIC-NEXT:   subs {{r[0-9]+}}, [[ORIGINAL_GUARD]], [[SAVED_GUARD]]
+;PIC-NEXT:   cmp [[ORIGINAL_GUARD]], [[SAVED_GUARD]]
 
 ;PIC:      [[GUARD_STACK_OFFSET]]:
 ;PIC-NEXT:   .long 1028
@@ -26,7 +26,7 @@
 ;NO-PIC-NOT: LPC
 ;NO-PIC-NEXT:           ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
 ;DYNAMIC-NO-PIC-NEXT:   ldr [[ORIGINAL_GUARD]], {{\[}}[[ORIGINAL_GUARD]]{{\]}}
-;NO-PIC-NEXT:           subs {{r[0-9]+}}, [[ORIGINAL_GUARD]], [[SAVED_GUARD]]
+;NO-PIC-NEXT:           cmp [[ORIGINAL_GUARD]], [[SAVED_GUARD]]
 
 ;STATIC:      [[GUARD_STACK_OFFSET]]:
 ;STATIC-NEXT:   .long 1028
Index: llvm/test/CodeGen/SystemZ/stack-guard.ll
===================================================================
--- llvm/test/CodeGen/SystemZ/stack-guard.ll
+++ llvm/test/CodeGen/SystemZ/stack-guard.ll
@@ -11,7 +11,7 @@
 ; CHECK: sllg [[REG2]], [[REG2]], 32
 ; CHECK: ear [[REG2]], %a1
 ; CHECK: lg [[REG2]], 40([[REG2]])
-; CHECK: sg [[REG2]], {{[0-9]*}}(%r15)
+; CHECK: cg [[REG2]], {{[0-9]*}}(%r15)
 
 define i32 @test_stack_guard() #0 {
 entry:
Index: llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
===================================================================
--- llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
+++ llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
@@ -2596,17 +2596,13 @@
                         MachineMemOperand::MOVolatile);
   }
 
-  // Perform the comparison via a subtract/getsetcc.
-  EVT VT = Guard.getValueType();
-  SDValue Sub = DAG.getNode(ISD::SUB, dl, VT, Guard, GuardVal);
-
+  // Perform the comparison via a getsetcc.
   SDValue Cmp = DAG.getSetCC(dl, TLI.getSetCCResultType(DAG.getDataLayout(),
                                                         *DAG.getContext(),
-                                                        Sub.getValueType()),
-                             Sub, DAG.getConstant(0, dl, VT), ISD::SETNE);
+                                                        Guard.getValueType()),
+                             Guard, GuardVal, ISD::SETNE);
 
-  // If the sub is not 0, then we know the guard/stackslot do not equal, so
-  // branch to failure MBB.
+  // If the guard/stackslot do not equal, branch to failure MBB.
   SDValue BrCond = DAG.getNode(ISD::BRCOND, dl,
                                MVT::Other, GuardVal.getOperand(0),
                                Cmp, DAG.getBasicBlock(SPD.getFailureMBB()));


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