[PATCH] D11680: [ARM] Be less conservative about forming IT blocks.

[James Molloy]

jmolloy created this revision.
jmolloy added reviewers: t.p.northover, MatzeB.
jmolloy added a subscriber: llvm-commits.
jmolloy set the repository for this revision to rL LLVM.
Herald added subscribers: rengolin, aemerson.

In http://reviews.llvm.org/rL215382, IT forming was made more conservative under
the belief that a flag-setting instruction was unpredictable inside an IT block on ARMv6M.

But actually, ARMv6M doesn't even support IT blocks so that's impossible. In the ARMARM for
v7M, v7AR and v8AR it states that the semantics of such an instruction changes inside an
IT block - it doesn't set the flags. So actually it is fine to use one inside an IT block
as long as the flags register is dead afterwards.

Repository:
  rL LLVM

http://reviews.llvm.org/D11680

Files:
  lib/Target/ARM/ARMBaseInstrInfo.cpp
  test/CodeGen/ARM/thumb2-it-block.ll

Index: test/CodeGen/ARM/thumb2-it-block.ll
===================================================================
--- test/CodeGen/ARM/thumb2-it-block.ll
+++ test/CodeGen/ARM/thumb2-it-block.ll
@@ -1,5 +1,5 @@
-; RUN: llc -mtriple=thumb-eabi -mcpu=arm1156t2-s -mattr=+thumb2 %s -o - | FileCheck -check-prefix CHECK-V7 %s
-; RUN: llc -mtriple=thumbv8 %s -o - | FileCheck %s -check-prefix CHECK-V8
+; RUN: llc -mtriple=thumb-eabi -mcpu=arm1156t2-s -mattr=+thumb2 %s -o - | FileCheck %s
+; RUN: llc -mtriple=thumbv8 %s -o - | FileCheck %s
 ; PR11107
 
 define i32 @test(i32 %a, i32 %b) {
@@ -14,17 +14,13 @@
  ret i32 %add
 }
 
-; CHECK-V7:        cmp
-; CHECK-V7-NEXT:   it    mi
-; CHECK-V7-NEXT:   rsbmi
-; CHECK-V7-NEXT:   cmp
-; CHECK-V7-NEXT:   it    mi
-; CHECK-V7-NEXT:   rsbmi
+; CHECK:        cmp
+; CHECK-NEXT:   it    mi
+; We shouldn't need to check for the extra 's' here; tRSB should be printed as
+; "rsb" inside an IT block, not "rsbs".
+; CHECK-NEXT:   rsb{{s?}}mi
+; CHECK-NEXT:   cmp
+; CHECK-NEXT:   it    mi
+; CHECK-NEXT:   rsb{{s?}}mi
 
-; CHECK-V8:        cmp
-; CHECK-V8-NEXT:   bpl
-; CHECK-V8:        rsbs
-; CHECK-V8:        cmp
-; CHECK-V8-NEXT:   bpl
-; CHECK-V8:        rsbs
 
Index: lib/Target/ARM/ARMBaseInstrInfo.cpp
===================================================================
--- lib/Target/ARM/ARMBaseInstrInfo.cpp
+++ lib/Target/ARM/ARMBaseInstrInfo.cpp
@@ -518,7 +518,7 @@
 
 static bool isCPSRDefined(const MachineInstr *MI) {
   for (const auto &MO : MI->operands())
-    if (MO.isReg() && MO.getReg() == ARM::CPSR && MO.isDef())
+    if (MO.isReg() && MO.getReg() == ARM::CPSR && MO.isDef() && !MO.isDead())
       return true;
   return false;
 }


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