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[AArch64][GlobalISel] Walk through G_TRUNC in getTestBitReg 

When you encounter a G_TRUNC, you are moving from a larger type to a smaller
type.

Asking for the i-th bit on a larger value is the same as asking for the i-th
bit on a smaller value.

So, we should always be able to walk through G_TRUNC when computing the bit
for a TB(N)Z.

Differential Revision: https://reviews.llvm.org/D73748
This commit is contained in:
Jessica Paquette 2020-01-30 14:53:37 -08:00
parent 574685b879
commit b9bf9305d1
2 changed files with 40 additions and 1 deletions
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6
llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
View File

@ -998,7 +998,11 @@ static Register getTestBitReg(Register Reg, MachineRegisterInfo &MRI) {
Register NextReg;
// (tbz (any_ext x), b) -> (tbz x, b) if we don't use the extended bits.
if (Opc == TargetOpcode::G_ANYEXT || Opc == TargetOpcode::G_ZEXT)
//
// (tbz (trunc x), b) -> (tbz x, b) is always safe, because the bit number
// on the truncated x is the same as the bit number on x.
if (Opc == TargetOpcode::G_ANYEXT || Opc == TargetOpcode::G_ZEXT ||
Opc == TargetOpcode::G_TRUNC)
NextReg = MI->getOperand(1).getReg();
// Did we find something worth folding?

35
llvm/test/CodeGen/AArch64/GlobalISel/opt-fold-trunc-tbz-tbnz.mir Normal file
View File

@ -0,0 +1,35 @@
# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py
# RUN: llc -mtriple aarch64-unknown-unknown -run-pass=instruction-select -verify-machineinstrs %s -o - | FileCheck %s
...
---
name: fold_trunc
alignment: 4
legalized: true
regBankSelected: true
tracksRegLiveness: true
body: |
; CHECK-LABEL: name: fold_trunc
; CHECK: bb.0:
; CHECK: successors: %bb.0(0x40000000), %bb.1(0x40000000)
; CHECK: liveins: $x0
; CHECK: %copy:gpr64all = COPY $x0
; CHECK: [[COPY:%[0-9]+]]:gpr32all = COPY %copy.sub_32
; CHECK: [[COPY1:%[0-9]+]]:gpr32 = COPY [[COPY]]
; CHECK: TBNZW [[COPY1]], 3, %bb.1
; CHECK: B %bb.0
; CHECK: bb.1:
; CHECK: RET_ReallyLR
bb.0:
successors: %bb.0, %bb.1
liveins: $x0
%copy:gpr(s64) = COPY $x0
%bit:gpr(s32) = G_CONSTANT i32 8
%zero:gpr(s32) = G_CONSTANT i32 0
%fold_me:gpr(s32) = G_TRUNC %copy(s64)
%and:gpr(s32) = G_AND %fold_me, %bit
%cmp:gpr(s32) = G_ICMP intpred(ne), %and(s32), %zero
%cmp_trunc:gpr(s1) = G_TRUNC %cmp(s32)
G_BRCOND %cmp_trunc(s1), %bb.1
G_BR %bb.0
bb.1:
RET_ReallyLR