[llvm] r289175 - [SCCP] Teach the pass about `mul %x 0` even if %x is overdefined.
Davide Italiano via llvm-commits
llvm-commits at lists.llvm.org
Thu Dec 8 19:08:43 PST 2016
Author: davide
Date: Thu Dec 8 21:08:42 2016
New Revision: 289175
URL: http://llvm.org/viewvc/llvm-project?rev=289175&view=rev
Log:
[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
Modified:
llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
llvm/trunk/test/Transforms/SCCP/logical-nuke.ll
Modified: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SCCP.cpp?rev=289175&r1=289174&r2=289175&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp Thu Dec 8 21:08:42 2016
@@ -916,7 +916,8 @@ void SCCPSolver::visitBinaryOperator(Ins
// 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(Ins
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 {
Modified: llvm/trunk/test/Transforms/SCCP/logical-nuke.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SCCP/logical-nuke.ll?rev=289175&r1=289174&r2=289175&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/SCCP/logical-nuke.ll (original)
+++ llvm/trunk/test/Transforms/SCCP/logical-nuke.ll Thu Dec 8 21:08:42 2016
@@ -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
+}
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