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Be less conservative about forming IT blocks.

Browse Source 
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.

This gives significant performance improvements in a variety of MPEG based workloads.

Differential revision: http://reviews.llvm.org/D11680

llvm-svn: 243869
This commit is contained in:
James Molloy 2015-08-03 09:24:48 +00:00
parent c58fe586e9
commit 08be907489
2 changed files with 11 additions and 15 deletions
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2
lib/Target/ARM/ARMBaseInstrInfo.cpp
View File

@ -518,7 +518,7 @@ bool ARMBaseInstrInfo::DefinesPredicate(MachineInstr *MI,
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;
}

24
test/CodeGen/ARM/thumb2-it-block.ll
View File

@ -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 @@ entry:
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