[X86] improve split-stack machine BB placement

Summary:
The conditional branch created to support -fsplit-stack for X86 is
left unbiased/unhinted, resulting in less than ideal block placement:
the __morestack call block is kept on the main hot path. Bias the
branch to insure that the stack allocation block is treated as a
"cold" block during machine basic block placement.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D54123

llvm-svn: 346336

llvm/lib/Target/X86/X86FrameLowering.cpp

@@ -2471,8 +2471,8 @@ void X86FrameLowering::adjustForSegmentedStacks(
 
   allocMBB->addSuccessor(&PrologueMBB);
 
-  checkMBB->addSuccessor(allocMBB);
-  checkMBB->addSuccessor(&PrologueMBB);
+  checkMBB->addSuccessor(allocMBB, BranchProbability::getZero());
+  checkMBB->addSuccessor(&PrologueMBB, BranchProbability::getOne());
 
 #ifdef EXPENSIVE_CHECKS
   MF.verify();

llvm/test/CodeGen/X86/segmented-stacks-dynamic.ll

@@ -25,12 +25,7 @@ false:
 ; X32-LABEL:      test_basic:
 
 ; X32:      cmpl %gs:48, %esp
-; X32-NEXT: ja      .LBB0_2
-
-; X32:      pushl $4
-; X32-NEXT: pushl $12
-; X32-NEXT: calll __morestack
-; X32-NEXT: ret
+; X32-NEXT: jbe	.LBB0_1
 
 ; X32:      movl %esp, %eax
 ; X32:      subl %ecx, %eax
@@ -43,15 +38,15 @@ false:
 ; X32-NEXT: calll __morestack_allocate_stack_space
 ; X32-NEXT: addl $16, %esp
 
+; X32:      pushl $4
+; X32-NEXT: pushl $12
+; X32-NEXT: calll __morestack
+; X32-NEXT: ret
+
 ; X64-LABEL:      test_basic:
 
 ; X64:      cmpq %fs:112, %rsp
-; X64-NEXT: ja      .LBB0_2
-
-; X64:      movabsq $24, %r10
-; X64-NEXT: movabsq $0, %r11
-; X64-NEXT: callq __morestack
-; X64-NEXT: ret
+; X64-NEXT: jbe      .LBB0_1
 
 ; X64:      movq %rsp, %[[RDI:rdi|rax]]
 ; X64:      subq %{{.*}}, %[[RDI]]
@@ -63,15 +58,15 @@ false:
 ; X64-NEXT: callq __morestack_allocate_stack_space
 ; X64:      movq %rax, %rdi
 
+; X64:      movabsq $24, %r10
+; X64-NEXT: movabsq $0, %r11
+; X64-NEXT: callq __morestack
+; X64-NEXT: ret
+
 ; X32ABI-LABEL:      test_basic:
 
 ; X32ABI:      cmpl %fs:64, %esp
-; X32ABI-NEXT: ja      .LBB0_2
-
-; X32ABI:      movl $24, %r10d
-; X32ABI-NEXT: movl $0, %r11d
-; X32ABI-NEXT: callq __morestack
-; X32ABI-NEXT: ret
+; X32ABI-NEXT: jbe      .LBB0_1
 
 ; X32ABI:      movl %esp, %[[EDI:edi|eax]]
 ; X32ABI:      subl %{{.*}}, %[[EDI]]
@@ -83,6 +78,11 @@ false:
 ; X32ABI-NEXT: callq __morestack_allocate_stack_space
 ; X32ABI:      movl %eax, %edi
 
+; X32ABI:      movl $24, %r10d
+; X32ABI-NEXT: movl $0, %r11d
+; X32ABI-NEXT: callq __morestack
+; X32ABI-NEXT: ret
+
 }
 
 attributes #0 = { "split-stack" }