[llvm] r269269 - [SCCP] Resolve shifts beyond the bitwidth to undef

[David Majnemer via llvm-commits]

Author: majnemer
Date: Wed May 11 22:07:40 2016
New Revision: 269269

URL: http://llvm.org/viewvc/llvm-project?rev=269269&view=rev
Log:
[SCCP] Resolve shifts beyond the bitwidth to undef

Shifts beyond the bitwidth are undef but SCCP resolved them to zero.
Instead, DTRT and resolve them to undef.

This reimplements the transform which caused PR27712.

Added:
    llvm/trunk/test/Transforms/SCCP/pr27712.ll
    llvm/trunk/test/Transforms/SCCP/ub-shift.ll
Modified:
    llvm/trunk/lib/Transforms/Scalar/SCCP.cpp

Modified: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Scalar/SCCP.cpp?rev=269269&r1=269268&r2=269269&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp (original)
+++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp Wed May 11 22:07:40 2016
@@ -1416,6 +1416,14 @@ bool SCCPSolver::ResolvedUndefsIn(Functi
         // X >>a undef -> undef.
         if (Op1LV.isUndefined()) break;
 
+        // Shifting by the bitwidth or more is undefined.
+        if (Op1LV.isConstant()) {
+          auto *ShiftAmt = Op1LV.getConstantInt();
+          if (ShiftAmt->getLimitedValue() >=
+              ShiftAmt->getType()->getScalarSizeInBits())
+            break;
+        }
+
         // undef >>a X -> all ones
         markForcedConstant(&I, Constant::getAllOnesValue(ITy));
         return true;
@@ -1425,6 +1433,14 @@ bool SCCPSolver::ResolvedUndefsIn(Functi
         // X >> undef -> undef.
         if (Op1LV.isUndefined()) break;
 
+        // Shifting by the bitwidth or more is undefined.
+        if (Op1LV.isConstant()) {
+          auto *ShiftAmt = Op1LV.getConstantInt();
+          if (ShiftAmt->getLimitedValue() >=
+              ShiftAmt->getType()->getScalarSizeInBits())
+            break;
+        }
+
         // undef << X -> 0
         // undef >> X -> 0
         markForcedConstant(&I, Constant::getNullValue(ITy));

Added: llvm/trunk/test/Transforms/SCCP/pr27712.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SCCP/pr27712.ll?rev=269269&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/SCCP/pr27712.ll (added)
+++ llvm/trunk/test/Transforms/SCCP/pr27712.ll Wed May 11 22:07:40 2016
@@ -0,0 +1,30 @@
+; RUN: opt -sccp -S < %s | FileCheck %s
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+define i32 @main() {
+entry:
+  br label %lbl_1154
+
+lbl_1154:
+  %b0.0 = phi i32 [ -119, %entry ], [ 0, %lbl_1154 ]
+  %cmp11 = icmp slt i32 %b0.0, 0
+  %shl.op = shl i32 33554432, %b0.0
+  %cmp1445 = icmp ult i32 %shl.op, 33554432
+  %cmp14 = or i1 %cmp11, %cmp1445
+  br i1 %cmp14, label %lbl_1154, label %if.end19
+
+if.end19:
+  br i1 %cmp11, label %if.then22, label %cleanup26
+
+if.then22:
+  tail call void @abort()
+  unreachable
+
+cleanup26:
+  ret i32 %shl.op
+}
+; CHECK-LABEL: define i32 @main(
+; CHECK-NOT: ret i32 undef
+
+declare void @abort()

Added: llvm/trunk/test/Transforms/SCCP/ub-shift.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/SCCP/ub-shift.ll?rev=269269&view=auto
==============================================================================
--- llvm/trunk/test/Transforms/SCCP/ub-shift.ll (added)
+++ llvm/trunk/test/Transforms/SCCP/ub-shift.ll Wed May 11 22:07:40 2016
@@ -0,0 +1,69 @@
+; RUN: opt < %s -sccp -S | FileCheck %s
+
+; CHECK-LABEL: shift_undef_64
+define void @shift_undef_64(i64* %p) {
+  %r1 = lshr i64 -1, 4294967296 ; 2^32
+  ; CHECK: store i64 undef
+  store i64 %r1, i64* %p
+
+  %r2 = ashr i64 -1, 4294967297 ; 2^32 + 1
+  ; CHECK: store i64 undef
+  store i64 %r2, i64* %p
+
+  %r3 = shl i64 -1, 4294967298 ; 2^32 + 2
+  ; CHECK: store i64 undef
+  store i64 %r3, i64* %p
+
+  ret void
+}
+
+; CHECK-LABEL: shift_undef_65
+define void @shift_undef_65(i65* %p) {
+  %r1 = lshr i65 2, 18446744073709551617
+  ; CHECK: store i65 undef
+  store i65 %r1, i65* %p
+
+  %r2 = ashr i65 4, 18446744073709551617
+  ; CHECK: store i65 undef
+  store i65 %r2, i65* %p
+
+  %r3 = shl i65 1, 18446744073709551617
+  ; CHECK: store i65 undef
+  store i65 %r3, i65* %p
+
+  ret void
+}
+
+; CHECK-LABEL: shift_undef_256
+define void @shift_undef_256(i256* %p) {
+  %r1 = lshr i256 2, 18446744073709551617
+  ; CHECK: store i256 undef
+  store i256 %r1, i256* %p
+
+  %r2 = ashr i256 4, 18446744073709551618
+  ; CHECK: store i256 undef
+  store i256 %r2, i256* %p
+
+  %r3 = shl i256 1, 18446744073709551619
+  ; CHECK: store i256 undef
+  store i256 %r3, i256* %p
+
+  ret void
+}
+
+; CHECK-LABEL: shift_undef_511
+define void @shift_undef_511(i511* %p) {
+  %r1 = lshr i511 -1, 1208925819614629174706276 ; 2^80 + 100
+  ; CHECK: store i511 undef
+  store i511 %r1, i511* %p
+
+  %r2 = ashr i511 -2, 1208925819614629174706200
+  ; CHECK: store i511 undef
+  store i511 %r2, i511* %p
+
+  %r3 = shl i511 -3, 1208925819614629174706180
+  ; CHECK: store i511 undef
+  store i511 %r3, i511* %p
+
+  ret void
+}