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[AArch64] Remove an overly conservative check when generating store pairs.

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Store instructions do not modify register values and therefore it's safe
to form a store pair even if the source register has been read in between
the two store instructions.

Previously, the read of w1 (see below) prevented the formation of a stp.

        str      w0, [x2]
        ldr     w8, [x2, #8]
        add      w0, w8, w1
        str     w1, [x2, #4]
        ret

We now generate the following code.

        stp      w0, w1, [x2]
        ldr     w8, [x2, #8]
        add      w0, w8, w1
        ret

All correctness tests with -Ofast on A57 with Spec200x and EEMBC pass.
Performance results for SPEC2K were within noise.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239432 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chad Rosier
2015-06-09 20:59:41 +00:00
parent a96fc7ab00
commit e2e26b486d
2 changed files with 35 additions and 2 deletions
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5
lib/Target/AArch64/AArch64LoadStoreOptimizer.cpp
View File

@@ -623,7 +623,7 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
// and first alias with the second, we can combine the second into the
// first.
if (!ModifiedRegs[MI->getOperand(0).getReg()] &&
!UsedRegs[MI->getOperand(0).getReg()] &&
!(MI->mayLoad() && UsedRegs[MI->getOperand(0).getReg()]) &&
!mayAlias(MI, MemInsns, TII)) {
MergeForward = false;
return MBBI;
@@ -634,7 +634,8 @@ AArch64LoadStoreOpt::findMatchingInsn(MachineBasicBlock::iterator I,
// first and the second alias with the first, we can combine the first
// into the second.
if (!ModifiedRegs[FirstMI->getOperand(0).getReg()] &&
!UsedRegs[FirstMI->getOperand(0).getReg()] &&
!(FirstMI->mayLoad() &&
UsedRegs[FirstMI->getOperand(0).getReg()]) &&
!mayAlias(FirstMI, MemInsns, TII)) {
MergeForward = true;
return MBBI;

32
test/CodeGen/AArch64/arm64-stp.ll
View File

@@ -99,3 +99,35 @@ entry:
store <4 x i32> %p20, <4 x i32>* %p21, align 4
ret void
}
; Read of %b to compute %tmp2 shouldn't prevent formation of stp
; CHECK-LABEL: stp_int_rar_hazard
; CHECK: stp w0, w1, [x2]
; CHECK: ldr [[REG:w[0-9]+]], [x2, #8]
; CHECK: add w0, [[REG]], w1
; CHECK: ret
define i32 @stp_int_rar_hazard(i32 %a, i32 %b, i32* nocapture %p) nounwind {
store i32 %a, i32* %p, align 4
%ld.ptr = getelementptr inbounds i32, i32* %p, i64 2
%tmp = load i32, i32* %ld.ptr, align 4
%tmp2 = add i32 %tmp, %b
%add.ptr = getelementptr inbounds i32, i32* %p, i64 1
store i32 %b, i32* %add.ptr, align 4
ret i32 %tmp2
}
; Read of %b to compute %tmp2 shouldn't prevent formation of stp
; CHECK-LABEL: stp_int_rar_hazard_after
; CHECK: ldr [[REG:w[0-9]+]], [x3, #4]
; CHECK: add w0, [[REG]], w2
; CHECK: stp w1, w2, [x3]
; CHECK: ret
define i32 @stp_int_rar_hazard_after(i32 %w0, i32 %a, i32 %b, i32* nocapture %p) nounwind {
store i32 %a, i32* %p, align 4
%ld.ptr = getelementptr inbounds i32, i32* %p, i64 1
%tmp = load i32, i32* %ld.ptr, align 4
%tmp2 = add i32 %tmp, %b
%add.ptr = getelementptr inbounds i32, i32* %p, i64 1
store i32 %b, i32* %add.ptr, align 4
ret i32 %tmp2
}