[PATCH] D81067: Fix dynamic probing scheme

[serge via Phabricator via llvm-commits]

serge-sans-paille created this revision.
serge-sans-paille added reviewers: craig.topper, jonpa.
Herald added subscribers: llvm-commits, hiraditya.
Herald added a project: LLVM.

If we probe *after* each static stack allocation, we need to probe *before* each
dynamic stack allocation. Provide a scheme to describe the possible scenario.

      

Thanks a lot to @jonpa for motivating this fix.


https://reviews.llvm.org/D81067

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/stack-clash-dynamic-alloca.ll


Index: llvm/test/CodeGen/X86/stack-clash-dynamic-alloca.ll
===================================================================
--- llvm/test/CodeGen/X86/stack-clash-dynamic-alloca.ll
+++ llvm/test/CodeGen/X86/stack-clash-dynamic-alloca.ll
@@ -26,8 +26,8 @@
 ; CHECK-X86-64-NEXT:  	cmpq	%rax, %rsp
 ; CHECK-X86-64-NEXT:  	jl	.LBB0_3
 ; CHECK-X86-64-NEXT:  .LBB0_2: # =>This Inner Loop Header: Depth=1
-; CHECK-X86-64-NEXT:  	subq	$4096, %rsp # imm = 0x1000
 ; CHECK-X86-64-NEXT:  	movq	$0, (%rsp)
+; CHECK-X86-64-NEXT:  	subq	$4096, %rsp # imm = 0x1000
 ; CHECK-X86-64-NEXT:  	cmpq	%rax, %rsp
 ; CHECK-X86-64-NEXT:  	jge	.LBB0_2
 ; CHECK-X86-64-NEXT:  .LBB0_3:
@@ -56,8 +56,8 @@
 ; CHECK-X86-32-NEXT:    cmpl    %eax, %esp
 ; CHECK-X86-32-NEXT:    jl  .LBB0_3
 ; CHECK-X86-32-NEXT:  .LBB0_2: # =>This Inner Loop Header: Depth=1
-; CHECK-X86-32-NEXT:    subl    $4096, %esp # imm = 0x1000
 ; CHECK-X86-32-NEXT:    movl    $0, (%esp)
+; CHECK-X86-32-NEXT:    subl    $4096, %esp # imm = 0x1000
 ; CHECK-X86-32-NEXT:    cmpl    %eax, %esp
 ; CHECK-X86-32-NEXT:    jge .LBB0_2
 ; CHECK-X86-32-NEXT:  .LBB0_3:
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -31650,17 +31650,28 @@
   testMBB->addSuccessor(blockMBB);
   testMBB->addSuccessor(tailMBB);
 
-  // allocate a block and touch it
+  // Touch the block then extend it. This is done on the opposite side of
+  // static probe where we allocate then touch, to avoid the need of probing the
+  // tail of the static alloca. Possible scenarios are:
+  //
+  //       + ---- <- ------------ <- ------------- <- ------------ +
+  //       |                                                       |
+  // [free probe] -> [page alloc] -> [alloc probe] -> [tail alloc] + -> [dyn probe] -> [page alloc] -> [dyn probe] -> [tail alloc] +
+  //                                                               |                                                               |
+  //                                                               + <- ----------- <- ------------ <- ----------- <- ------------ +
+  //
+  // The property we want to enforce is to never have more than [page alloc] between two probes.
+
+  const unsigned MovMIOpc =
+      TFI.Uses64BitFramePtr ? X86::MOV64mi32 : X86::MOV32mi;
+  addRegOffset(BuildMI(blockMBB, DL, TII->get(MovMIOpc)), physSPReg, false, 0)
+      .addImm(0);
 
   BuildMI(blockMBB, DL,
           TII->get(getSUBriOpcode(TFI.Uses64BitFramePtr, ProbeSize)), physSPReg)
       .addReg(physSPReg)
       .addImm(ProbeSize);
 
-  const unsigned MovMIOpc =
-      TFI.Uses64BitFramePtr ? X86::MOV64mi32 : X86::MOV32mi;
-  addRegOffset(BuildMI(blockMBB, DL, TII->get(MovMIOpc)), physSPReg, false, 0)
-      .addImm(0);
 
   BuildMI(blockMBB, DL, TII->get(X86::JMP_1)).addMBB(testMBB);
   blockMBB->addSuccessor(testMBB);


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