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When fast iseling a GEP, accumulate the offset rather than emitting a series of

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ADDs.  MaxOffs is used as a threshold to limit the size of the offset. Tradeoffs
being: (1) If we can't materialize the large constant then we'll cause fast-isel
to bail. (2) Too large of an offset can't be directly encoded in the ADD
resulting in a MOV+ADD.  Generally not a bad thing because otherwise we would
have had ADD+ADD, but on Thumb this turns into a MOVS+MOVT+ADD. Working on a fix
for that. (3) Conversely, too low of a threshold we'll miss opportunities to 
coalesce ADDs.
rdar://10412592

llvm-svn: 144886
This commit is contained in:
Chad Rosier 2011-11-17 07:15:58 +00:00
parent 4d39196041
commit 2673f8862f
2 changed files with 100 additions and 11 deletions
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36
lib/CodeGen/SelectionDAG/FastISel.cpp
View File

@ -437,6 +437,11 @@ bool FastISel::SelectGetElementPtr(const User *I) {
bool NIsKill = hasTrivialKill(I->getOperand(0));
// Keep a running tab of the total offset to coalesce multiple N = N + Offset
// into a single N = N + TotalOffset.
uint64_t TotalOffs = 0;
// FIXME: What's a good SWAG number for MaxOffs?
uint64_t MaxOffs = 2048;
Type *Ty = I->getOperand(0)->getType();
MVT VT = TLI.getPointerTy();
for (GetElementPtrInst::const_op_iterator OI = I->op_begin()+1,
@ -446,14 +451,15 @@ bool FastISel::SelectGetElementPtr(const User *I) {
unsigned Field = cast<ConstantInt>(Idx)->getZExtValue();
if (Field) {
// N = N + Offset
uint64_t Offs = TD.getStructLayout(StTy)->getElementOffset(Field);
// FIXME: This can be optimized by combining the add with a
// subsequent one.
N = FastEmit_ri_(VT, ISD::ADD, N, NIsKill, Offs, VT);
TotalOffs += TD.getStructLayout(StTy)->getElementOffset(Field);
if (TotalOffs >= MaxOffs) {
N = FastEmit_ri_(VT, ISD::ADD, N, NIsKill, TotalOffs, VT);
if (N == 0)
// Unhandled operand. Halt "fast" selection and bail.
return false;
NIsKill = true;
TotalOffs = 0;
}
}
Ty = StTy->getElementType(Field);
} else {
@ -462,15 +468,27 @@ bool FastISel::SelectGetElementPtr(const User *I) {
// If this is a constant subscript, handle it quickly.
if (const ConstantInt *CI = dyn_cast<ConstantInt>(Idx)) {
if (CI->isZero()) continue;
uint64_t Offs =
// N = N + Offset
TotalOffs +=
TD.getTypeAllocSize(Ty)*cast<ConstantInt>(CI)->getSExtValue();
N = FastEmit_ri_(VT, ISD::ADD, N, NIsKill, Offs, VT);
if (TotalOffs >= MaxOffs) {
N = FastEmit_ri_(VT, ISD::ADD, N, NIsKill, TotalOffs, VT);
if (N == 0)
// Unhandled operand. Halt "fast" selection and bail.
return false;
NIsKill = true;
TotalOffs = 0;
}
continue;
}
if (TotalOffs) {
N = FastEmit_ri_(VT, ISD::ADD, N, NIsKill, TotalOffs, VT);
if (N == 0)
// Unhandled operand. Halt "fast" selection and bail.
return false;
NIsKill = true;
TotalOffs = 0;
}
// N = N + Idx * ElementSize;
uint64_t ElementSize = TD.getTypeAllocSize(Ty);
@ -494,6 +512,12 @@ bool FastISel::SelectGetElementPtr(const User *I) {
return false;
}
}
if (TotalOffs) {
N = FastEmit_ri_(VT, ISD::ADD, N, NIsKill, TotalOffs, VT);
if (N == 0)
// Unhandled operand. Halt "fast" selection and bail.
return false;
}
// We successfully emitted code for the given LLVM Instruction.
UpdateValueMap(I, N);

65
test/CodeGen/ARM/fast-isel-GEP-coalesce.ll Normal file
View File

@ -0,0 +1,65 @@
; RUN: llc < %s -O0 -fast-isel-abort -relocation-model=dynamic-no-pic -mtriple=armv7-apple-darwin | FileCheck %s --check-prefix=ARM
; RUN: llc < %s -O0 -fast-isel-abort -relocation-model=dynamic-no-pic -mtriple=thumbv7-apple-darwin | FileCheck %s --check-prefix=THUMB
%struct.A = type { i32, [2 x [2 x i32]], i8, [3 x [3 x [3 x i32]]] }
%struct.B = type { i32, [2 x [2 x [2 x %struct.A]]] }
@arr = common global [2 x [2 x [2 x [2 x [2 x i32]]]]] zeroinitializer, align 4
@A = common global [3 x [3 x %struct.A]] zeroinitializer, align 4
@B = common global [2 x [2 x [2 x %struct.B]]] zeroinitializer, align 4
define i32* @t1() nounwind {
entry:
; ARM: t1
; THUMB: t1
%addr = alloca i32*, align 4
store i32* getelementptr inbounds ([2 x [2 x [2 x [2 x [2 x i32]]]]]* @arr, i32 0, i32 1, i32 1, i32 1, i32 1, i32 1), i32** %addr, align 4
; ARM: add r0, r0, #124
; THUMB: adds r0, #124
%0 = load i32** %addr, align 4
ret i32* %0
}
define i32* @t2() nounwind {
entry:
; ARM: t2
; THUMB: t2
%addr = alloca i32*, align 4
store i32* getelementptr inbounds ([3 x [3 x %struct.A]]* @A, i32 0, i32 2, i32 2, i32 3, i32 1, i32 2, i32 2), i32** %addr, align 4
; ARM: movw r1, #1148
; ARM: add r0, r0, r1
; THUMB: addw r0, r0, #1148
%0 = load i32** %addr, align 4
ret i32* %0
}
define i32* @t3() nounwind {
entry:
; ARM: t3
; THUMB: t3
%addr = alloca i32*, align 4
store i32* getelementptr inbounds ([3 x [3 x %struct.A]]* @A, i32 0, i32 0, i32 1, i32 1, i32 0, i32 1), i32** %addr, align 4
; ARM: add r0, r0, #140
; THUMB: adds r0, #140
%0 = load i32** %addr, align 4
ret i32* %0
}
define i32* @t4() nounwind {
entry:
; ARM: t4
; THUMB: t4
%addr = alloca i32*, align 4
store i32* getelementptr inbounds ([2 x [2 x [2 x %struct.B]]]* @B, i32 0, i32 0, i32 0, i32 1, i32 1, i32 0, i32 0, i32 1, i32 3, i32 1, i32 2, i32 1), i32** %addr, align 4
; ARM-NOT: movw r{{[0-9]}}, #1060
; ARM-NOT: add r{{[0-9]}}, r{{[0-9]}}, #4
; ARM-NOT: add r{{[0-9]}}, r{{[0-9]}}, #132
; ARM-NOT: add r{{[0-9]}}, r{{[0-9]}}, #24
; ARM-NOT: add r{{[0-9]}}, r{{[0-9]}}, #36
; ARM-NOT: add r{{[0-9]}}, r{{[0-9]}}, #24
; ARM-NOT: add r{{[0-9]}}, r{{[0-9]}}, #4
; ARM: movw r{{[0-9]}}, #1284
; THUMB: addw r{{[0-9]}}, r{{[0-9]}}, #1284
%0 = load i32** %addr, align 4
ret i32* %0
}