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Extend known bits to understand @llvm.bswap

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This is a cleaned up patch from the one written by John Regehr based on the findings of the Souper superoptimizer.

When writing tests, I was surprised to find that instsimplify apparently doesn't know how to collapse bit test sequences based purely on known bits. This required me to split my tests across both instsimplify and instcombine.

Differential Revision: http://reviews.llvm.org/D13250

llvm-svn: 249453
This commit is contained in:
Philip Reames 2015-10-06 20:20:45 +00:00
parent dd55dd6aa8
commit ca99cf7d94
3 changed files with 94 additions and 0 deletions
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6
lib/Analysis/ValueTracking.cpp
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@ -1358,6 +1358,12 @@ static void computeKnownBitsFromOperator(Operator *I, APInt &KnownZero,
if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I)) {
switch (II->getIntrinsicID()) {
default: break;
case Intrinsic::bswap:
computeKnownBits(I->getOperand(0), KnownZero2, KnownOne2, DL,
Depth + 1, Q);
KnownZero |= KnownZero2.byteSwap();
KnownOne |= KnownOne2.byteSwap();
break;
case Intrinsic::ctlz:
case Intrinsic::cttz: {
unsigned LowBits = Log2_32(BitWidth)+1;

47
test/Transforms/InstCombine/bswap-known-bits.ll Normal file
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@ -0,0 +1,47 @@
; RUN: opt < %s -S -instcombine | FileCheck %s
; Note: This is testing functionality in computeKnownBits. I'd have rather
; used instsimplify, but the bit test folding is apparently only in instcombine.
declare i16 @llvm.bswap.i16(i16)
declare i32 @llvm.bswap.i32(i32)
define i1 @test1(i16 %arg) {
; CHECK-LABEL: @test1
; CHECK: ret i1 true
%a = or i16 %arg, 511
%b = call i16 @llvm.bswap.i16(i16 %a)
%and = and i16 %b, 256
%res = icmp eq i16 %and, 256
ret i1 %res
}
define i1 @test2(i16 %arg) {
; CHECK-LABEL: @test2
; CHECK: ret i1 true
%a = or i16 %arg, 1
%b = call i16 @llvm.bswap.i16(i16 %a)
%and = and i16 %b, 256
%res = icmp eq i16 %and, 256
ret i1 %res
}
define i1 @test3(i16 %arg) {
; CHECK-LABEL: @test3
; CHECK: ret i1 true
%a = or i16 %arg, 256
%b = call i16 @llvm.bswap.i16(i16 %a)
%and = and i16 %b, 1
%res = icmp eq i16 %and, 1
ret i1 %res
}
define i1 @test4(i32 %arg) {
; CHECK-LABEL: @test4
; CHECK: ret i1 true
%a = or i32 %arg, 2147483647 ; i32_MAX
%b = call i32 @llvm.bswap.i32(i32 %a)
%and = and i32 %b, 127
%res = icmp eq i32 %and, 127
ret i1 %res
}

41
test/Transforms/InstSimplify/bswap.ll Normal file
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@ -0,0 +1,41 @@
; RUN: opt < %s -S -instsimplify | FileCheck %s
declare i16 @llvm.bswap.i16(i16)
define i1 @test1(i16 %arg) {
; CHECK-LABEL: @test1
; CHECK: ret i1 false
%a = or i16 %arg, 1
%b = call i16 @llvm.bswap.i16(i16 %a)
%res = icmp eq i16 %b, 0
ret i1 %res
}
define i1 @test2(i16 %arg) {
; CHECK-LABEL: @test2
; CHECK: ret i1 false
%a = or i16 %arg, 1024
%b = call i16 @llvm.bswap.i16(i16 %a)
%res = icmp eq i16 %b, 0
ret i1 %res
}
define i1 @test3(i16 %arg) {
; CHECK-LABEL: @test3
; CHECK: ret i1 false
%a = and i16 %arg, 1
%b = call i16 @llvm.bswap.i16(i16 %a)
%and = and i16 %b, 1
%res = icmp eq i16 %and, 1
ret i1 %res
}
define i1 @test4(i16 %arg) {
; CHECK-LABEL: @test4
; CHECK: ret i1 false
%a = and i16 %arg, 511
%b = call i16 @llvm.bswap.i16(i16 %a)
%and = and i16 %b, 256
%res = icmp eq i16 %and, 1
ret i1 %res
}