ARM: VIXL32: Improve codegen on CBZ/CBNZ
This patch improves codegen on CBZ/CBNZ.
CompareAndBranchIfZero/CompareAndBranchIfNonZero are introduced.
These two functions can generate CMP+Bcc or Cbz/Cbnz.
CMP+Bcc are generated by default.
If a hint is given (is_far_target = false) and rn and label
can all fit into Cbz/Cbnz, then Cbz/Cbnz is generated.
Prefer these two interfaces to using vixl32::MacroAssembler::Cbz/Cbnz.
In T32, Cbz/Cbnz instructions have following limitations:
- Far targets, which are over 126 bytes away, are not supported.
- Only low registers can be encoded.
- Backward branches are not supported.
Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-host
Test: ART_USE_VIXL_ARM_BACKEND=true m test-art-target
Change-Id: I5d2ada19ea4f83dab78baf0cf78c72e99e58d946
diff --git a/compiler/utils/arm/assembler_arm_vixl.cc b/compiler/utils/arm/assembler_arm_vixl.cc
index e3b9fb6..1aaa231 100644
--- a/compiler/utils/arm/assembler_arm_vixl.cc
+++ b/compiler/utils/arm/assembler_arm_vixl.cc
@@ -429,5 +429,31 @@
}
}
+void ArmVIXLMacroAssembler::CompareAndBranchIfZero(vixl32::Register rn,
+ vixl32::Label* label,
+ bool is_far_target) {
+ if (!is_far_target && rn.IsLow() && !label->IsBound()) {
+ // In T32, Cbz/Cbnz instructions have following limitations:
+ // - There are only 7 bits (i:imm5:0) to encode branch target address (cannot be far target).
+ // - Only low registers (i.e R0 .. R7) can be encoded.
+ // - Only forward branches (unbound labels) are supported.
+ Cbz(rn, label);
+ return;
+ }
+ Cmp(rn, 0);
+ B(eq, label);
+}
+
+void ArmVIXLMacroAssembler::CompareAndBranchIfNonZero(vixl32::Register rn,
+ vixl32::Label* label,
+ bool is_far_target) {
+ if (!is_far_target && rn.IsLow() && !label->IsBound()) {
+ Cbnz(rn, label);
+ return;
+ }
+ Cmp(rn, 0);
+ B(ne, label);
+}
+
} // namespace arm
} // namespace art
diff --git a/compiler/utils/arm/assembler_arm_vixl.h b/compiler/utils/arm/assembler_arm_vixl.h
index e020628..b4a4abc 100644
--- a/compiler/utils/arm/assembler_arm_vixl.h
+++ b/compiler/utils/arm/assembler_arm_vixl.h
@@ -37,6 +37,25 @@
namespace art {
namespace arm {
+class ArmVIXLMacroAssembler FINAL : public vixl32::MacroAssembler {
+ public:
+ // The following interfaces can generate CMP+Bcc or Cbz/Cbnz.
+ // CMP+Bcc are generated by default.
+ // If a hint is given (is_far_target = false) and rn and label can all fit into Cbz/Cbnz,
+ // then Cbz/Cbnz is generated.
+ // Prefer following interfaces to using vixl32::MacroAssembler::Cbz/Cbnz.
+ // In T32, Cbz/Cbnz instructions have following limitations:
+ // - Far targets, which are over 126 bytes away, are not supported.
+ // - Only low registers can be encoded.
+ // - Backward branches are not supported.
+ void CompareAndBranchIfZero(vixl32::Register rn,
+ vixl32::Label* label,
+ bool is_far_target = true);
+ void CompareAndBranchIfNonZero(vixl32::Register rn,
+ vixl32::Label* label,
+ bool is_far_target = true);
+};
+
class ArmVIXLAssembler FINAL : public Assembler {
private:
class ArmException;
@@ -48,7 +67,7 @@
}
virtual ~ArmVIXLAssembler() {}
- vixl32::MacroAssembler* GetVIXLAssembler() { return &vixl_masm_; }
+ ArmVIXLMacroAssembler* GetVIXLAssembler() { return &vixl_masm_; }
void FinalizeCode() OVERRIDE;
// Size of generated code.
@@ -117,7 +136,7 @@
private:
// VIXL assembler.
- vixl32::MacroAssembler vixl_masm_;
+ ArmVIXLMacroAssembler vixl_masm_;
};
// Thread register declaration.
diff --git a/compiler/utils/arm/jni_macro_assembler_arm_vixl.cc b/compiler/utils/arm/jni_macro_assembler_arm_vixl.cc
index 23b2774..b2bbd72 100644
--- a/compiler/utils/arm/jni_macro_assembler_arm_vixl.cc
+++ b/compiler/utils/arm/jni_macro_assembler_arm_vixl.cc
@@ -574,10 +574,12 @@
switch (condition) {
case JNIMacroUnaryCondition::kZero:
- ___ Cbz(test.AsArm().AsVIXLRegister(), ArmVIXLJNIMacroLabel::Cast(label)->AsArm());
+ ___ CompareAndBranchIfZero(test.AsArm().AsVIXLRegister(),
+ ArmVIXLJNIMacroLabel::Cast(label)->AsArm());
break;
case JNIMacroUnaryCondition::kNotZero:
- ___ Cbnz(test.AsArm().AsVIXLRegister(), ArmVIXLJNIMacroLabel::Cast(label)->AsArm());
+ ___ CompareAndBranchIfNonZero(test.AsArm().AsVIXLRegister(),
+ ArmVIXLJNIMacroLabel::Cast(label)->AsArm());
break;
default:
LOG(FATAL) << "Not implemented unary condition: " << static_cast<int>(condition);