[SCCP] Teach the pass about `mul %x 0` even if %x is overdefined.

The motivating example is:

extern int patatino;
int goo() {
    int x = 0;
    for (int i = 0; i < 1000000; ++i) {
        x *= patatino;
    }
    return x;
}

Currently SCCP will not realize that this function returns always zero,
therefore will try to unroll and vectorize the loop at -O3 producing an
awful lot of (useless) code. With this change, it will just produce:

0000000000000000 <g>:
   xor    %eax,%eax
   retq

llvm-svn: 289175
This commit is contained in:
Davide Italiano 2016-12-09 03:08:42 +00:00
parent e3a0aef2cf
commit 824d695231
2 changed files with 13 additions and 3 deletions

View File

@ -916,7 +916,8 @@ void SCCPSolver::visitBinaryOperator(Instruction &I) {
// If this is an AND or OR with 0 or -1, it doesn't matter that the other
// operand is overdefined.
if (I.getOpcode() == Instruction::And || I.getOpcode() == Instruction::Or) {
if (I.getOpcode() == Instruction::And || I.getOpcode() == Instruction::Mul ||
I.getOpcode() == Instruction::Or) {
LatticeVal *NonOverdefVal = nullptr;
if (!V1State.isOverdefined())
NonOverdefVal = &V1State;
@ -927,8 +928,10 @@ void SCCPSolver::visitBinaryOperator(Instruction &I) {
if (NonOverdefVal->isUnknown())
return;
if (I.getOpcode() == Instruction::And) {
if (I.getOpcode() == Instruction::And ||
I.getOpcode() == Instruction::Mul) {
// X and 0 = 0
// X * 0 = 0
if (NonOverdefVal->getConstant()->isNullValue())
return markConstant(IV, &I, NonOverdefVal->getConstant());
} else {

View File

@ -1,6 +1,6 @@
; RUN: opt < %s -sccp -S | FileCheck %s
; Test that SCCP has basic knowledge of when and/or nuke overdefined values.
; Test that SCCP has basic knowledge of when and/or/mul nuke overdefined values.
; CHECK-LABEL: test
; CHECK: ret i32 0
@ -29,3 +29,10 @@ define i32 @test4(i32 %X) {
%Y = or i32 %X, undef
ret i32 %Y
}
; CHECK-LABEL: test5
; CHECK: ret i32 0
define i32 @test5(i32 %foo) {
%patatino = mul i32 %foo, undef
ret i32 %patatino
}