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[InstCombine] When canonicalizing gep indices, prefer zext when possible

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If we know that the sign bit of a value being sign extended is zero, we can use a zero extension instead.  This is motivated by the fact that zero extensions are generally cheaper on x86 (and most other architectures?).  We already apply a similar transform in DAGCombine, this just extends that to the IR level.

This comes up when we eagerly canonicalize gep indices to the width of a machine register (i64 on x86_64). To do so, we insert sign extensions (sext) to promote smaller types. 

Differential Revision: http://reviews.llvm.org/D7255



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229189 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Philip Reames 2015-02-14 00:05:36 +00:00
parent c4300b9c01
commit d777c2c0c0
2 changed files with 70 additions and 0 deletions
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9
lib/Transforms/InstCombine/InstCombineCasts.cpp
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@ -1064,6 +1064,15 @@ Instruction *InstCombiner::visitSExt(SExtInst &CI) {
Value *Src = CI.getOperand(0);
Type *SrcTy = Src->getType(), *DestTy = CI.getType();
// If we know that the value being extended is positive, we can use a zext
// instead.
bool KnownZero, KnownOne;
ComputeSignBit(Src, KnownZero, KnownOne, 0, &CI);
if (KnownZero) {
Value *ZExt = Builder->CreateZExt(Src, DestTy);
return ReplaceInstUsesWith(CI, ZExt);
}
// Attempt to extend the entire input expression tree to the destination
// type. Only do this if the dest type is a simple type, don't convert the
// expression tree to something weird like i93 unless the source is also

61
test/Transforms/InstCombine/gep-sext.ll Normal file
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@ -0,0 +1,61 @@
; RUN: opt < %s -instcombine -S | FileCheck %s
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
target triple = "x86_64-pc-win32"
declare void @use(i32) readonly
; We prefer to canonicalize the machine width gep indices early
define void @test(i32* %p, i32 %index) {
; CHECK-LABEL: @test
; CHECK-NEXT: %1 = sext i32 %index to i64
; CHECK-NEXT: %addr = getelementptr i32* %p, i64 %1
%addr = getelementptr i32* %p, i32 %index
%val = load i32* %addr
call void @use(i32 %val)
ret void
}
; If they've already been canonicalized via zext, that's fine
define void @test2(i32* %p, i32 %index) {
; CHECK-LABEL: @test2
; CHECK-NEXT: %i = zext i32 %index to i64
; CHECK-NEXT: %addr = getelementptr i32* %p, i64 %i
%i = zext i32 %index to i64
%addr = getelementptr i32* %p, i64 %i
%val = load i32* %addr
call void @use(i32 %val)
ret void
}
; If we can use a zext, we prefer that. This requires
; knowing that the index is positive.
define void @test3(i32* %p, i32 %index) {
; CHECK-LABEL: @test3
; CHECK: zext
; CHECK-NOT: sext
%addr_begin = getelementptr i32* %p, i64 40
%addr_fixed = getelementptr i32* %addr_begin, i64 48
%val_fixed = load i32* %addr_fixed, !range !0
%addr = getelementptr i32* %addr_begin, i32 %val_fixed
%val = load i32* %addr
call void @use(i32 %val)
ret void
}
; Replace sext with zext where possible
define void @test4(i32* %p, i32 %index) {
; CHECK-LABEL: @test4
; CHECK: zext
; CHECK-NOT: sext
%addr_begin = getelementptr i32* %p, i64 40
%addr_fixed = getelementptr i32* %addr_begin, i64 48
%val_fixed = load i32* %addr_fixed, !range !0
%i = sext i32 %val_fixed to i64
%addr = getelementptr i32* %addr_begin, i64 %i
%val = load i32* %addr
call void @use(i32 %val)
ret void
}
;; !range !0
!0 = !{i32 0, i32 2147483647}