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[SCCP] Teach the pass how to handle `div` with overdefined operands. 

This can prove that:

extern int f;
int g() {
    int x = 0;
    for (int i = 0; i < 365; ++i) {
        x /= f;
    }
    return x;
}

always returns zero. Thanks to Sanjoy for confirming this
transformation actually made sense (bugs are mine).

llvm-svn: 292531
This commit is contained in:
Davide Italiano 2017-01-19 23:07:51 +00:00
parent 5a52af0f63
commit 6c2c3e07bf
2 changed files with 38 additions and 0 deletions
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6
llvm/lib/Transforms/Scalar/SCCP.cpp
View File

@ -910,6 +910,12 @@ void SCCPSolver::visitBinaryOperator(Instruction &I) {
// Otherwise, one of our operands is overdefined. Try to produce something // Otherwise, one of our operands is overdefined. Try to produce something
// better than overdefined with some tricks. // better than overdefined with some tricks.
// If this is 0 / Y, it doesn't matter that the second operand is
// overdefined, and we can replace it with zero.
if (I.getOpcode() == Instruction::UDiv || I.getOpcode() == Instruction::SDiv)
if (V1State.isConstant() && V1State.getConstant()->isNullValue())
return markConstant(IV, &I, V1State.getConstant());
// If this is: // If this is:
// -> AND/MUL with 0 // -> AND/MUL with 0
// -> OR with -1 // -> OR with -1

32
llvm/test/Transforms/SCCP/overdefined-div.ll Normal file
View File

@ -0,0 +1,32 @@
; RUN: opt < %s -sccp -S | FileCheck %s
; Test that SCCP has basic knowledge of when div can nuke overdefined values.
; 0 / X = 0 even if X is overdefined.
; CHECK-LABEL: test1
; CHECK-NEXT: ret i32 0
define i32 @test1(i32 %foo) {
%tinkywinky = udiv i32 0, %foo
ret i32 %tinkywinky
}
; CHECK-LABEL: test2
; CHECK-NEXT: ret i32 0
define i32 @test2(i32 %foo) {
%tinkywinky = sdiv i32 0, %foo
ret i32 %tinkywinky
}
; CHECK-LABEL: test3
; CHECK: ret i32 %tinkywinky
define i32 @test3(i32 %foo) {
%tinkywinky = udiv i32 %foo, 0
ret i32 %tinkywinky
}
; CHECK-LABEL: test4
; CHECK: ret i32 %tinkywinky
define i32 @test4(i32 %foo) {
%tinkywinky = sdiv i32 %foo, 0
ret i32 %tinkywinky
}