[llvm] 29f189f - [InstCombine] Conditionally emit nowrap flags when combining two adds

[Nikita Popov via llvm-commits]

Author: Mehrnoosh Heidarpour
Date: 2021-04-14T20:53:06+02:00
New Revision: 29f189f90d44a021dafc859b0db4def53b1b46a3

URL: https://github.com/llvm/llvm-project/commit/29f189f90d44a021dafc859b0db4def53b1b46a3
DIFF: https://github.com/llvm/llvm-project/commit/29f189f90d44a021dafc859b0db4def53b1b46a3.diff

LOG: [InstCombine] Conditionally emit nowrap flags when combining two adds

Currently, the InstCombineCompare is combining two add operations
into a single add operation which always has a nsw flag, without
checking the conditions to see if this flag should be present
according to the original two add operations or not.

This patch will change the InstCombineCompare to emit the nsw or
nuw only when these flags are allowed to be generated according to
the original add operations and remove the possibility of applying
wrong optimization with passes that will perform on the IR later
in the pipeline.

To confirm that the current results are buggy and the results after
proposed patch are the correct IR the following examples from Alive2
are attached; the same results can be seen in the case of nuw flag
and nsw is just used as an example. The following link shows that
the generated IR with current LLVM is a buggy IR when none of the
original add operations have nsw flag.
https://alive2.llvm.org/ce/z/WGaDrm
The following link proves that the generated IR after the patch in
the former case is the correct IR.
https://alive2.llvm.org/ce/z/wQ7G_e

Differential Revision: https://reviews.llvm.org/D100095

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
    llvm/test/Transforms/InstCombine/icmp-add.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 147483084395..fda87286fb38 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3917,11 +3917,15 @@ Instruction *InstCombinerImpl::foldICmpBinOp(ICmpInst &I,
           APInt AP2Abs = C2->getValue().abs();
           if (AP1Abs.uge(AP2Abs)) {
             ConstantInt *C3 = Builder.getInt(AP1 - AP2);
-            Value *NewAdd = Builder.CreateNSWAdd(A, C3);
+            bool HasNUW = BO0->hasNoUnsignedWrap() && C3->getValue().ule(AP1);
+            bool HasNSW = BO0->hasNoSignedWrap();
+            Value *NewAdd = Builder.CreateAdd(A, C3, "", HasNUW, HasNSW);
             return new ICmpInst(Pred, NewAdd, C);
           } else {
             ConstantInt *C3 = Builder.getInt(AP2 - AP1);
-            Value *NewAdd = Builder.CreateNSWAdd(C, C3);
+            bool HasNUW = BO1->hasNoUnsignedWrap() && C3->getValue().ule(AP2);
+            bool HasNSW = BO1->hasNoSignedWrap();
+            Value *NewAdd = Builder.CreateAdd(C, C3, "", HasNUW, HasNSW);
             return new ICmpInst(Pred, A, NewAdd);
           }
         }

diff  --git a/llvm/test/Transforms/InstCombine/icmp-add.ll b/llvm/test/Transforms/InstCombine/icmp-add.ll
index c330cbc202ae..a35d546f23ae 100644
--- a/llvm/test/Transforms/InstCombine/icmp-add.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-add.ll
@@ -666,7 +666,7 @@ define <2 x i1> @icmp_eq_add_non_splat2(<2 x i32> %a) {
 
 define i1 @without_nsw_nuw(i8 %x, i8 %y) {
 ; CHECK-LABEL: @without_nsw_nuw(
-; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i8 [[X:%.*]], 2
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[X:%.*]], 2
 ; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8 [[TMP1]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[TOBOOL]]
 ;
@@ -678,7 +678,7 @@ define i1 @without_nsw_nuw(i8 %x, i8 %y) {
 
 define i1 @with_nsw_nuw(i8 %x, i8 %y) {
 ; CHECK-LABEL: @with_nsw_nuw(
-; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i8 [[X:%.*]], 2
+; CHECK-NEXT:    [[TMP1:%.*]] = add nuw nsw i8 [[X:%.*]], 2
 ; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8 [[TMP1]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[TOBOOL]]
 ;
@@ -702,7 +702,7 @@ define i1 @with_nsw_large(i8 %x, i8 %y) {
 
 define i1 @with_nsw_small(i8 %x, i8 %y) {
 ; CHECK-LABEL: @with_nsw_small(
-; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i8 [[Y:%.*]], 2
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[Y:%.*]], 2
 ; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8 [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret i1 [[TOBOOL]]
 ;
@@ -714,7 +714,7 @@ define i1 @with_nsw_small(i8 %x, i8 %y) {
 
 define i1 @with_nuw_large(i8 %x, i8 %y) {
 ; CHECK-LABEL: @with_nuw_large(
-; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i8 [[X:%.*]], 2
+; CHECK-NEXT:    [[TMP1:%.*]] = add nuw i8 [[X:%.*]], 2
 ; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8 [[TMP1]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[TOBOOL]]
 ;
@@ -726,7 +726,7 @@ define i1 @with_nuw_large(i8 %x, i8 %y) {
 
 define i1 @with_nuw_small(i8 %x, i8 %y) {
 ; CHECK-LABEL: @with_nuw_small(
-; CHECK-NEXT:    [[TMP1:%.*]] = add nsw i8 [[Y:%.*]], 2
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[Y:%.*]], 2
 ; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8 [[TMP1]], [[X:%.*]]
 ; CHECK-NEXT:    ret i1 [[TOBOOL]]
 ;
@@ -735,3 +735,15 @@ define i1 @with_nuw_small(i8 %x, i8 %y) {
   %tobool = icmp eq i8 %t2, %t1
   ret i1 %tobool
 }
+
+define i1 @with_nuw_large_negative(i8 %x, i8 %y) {
+; CHECK-LABEL: @with_nuw_large_negative(
+; CHECK-NEXT:    [[TMP1:%.*]] = add i8 [[X:%.*]], -2
+; CHECK-NEXT:    [[TOBOOL:%.*]] = icmp eq i8 [[TMP1]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[TOBOOL]]
+;
+  %t1 = add nuw i8 %x, -37
+  %t2 = add i8 %y, -35
+  %tobool = icmp eq i8 %t2, %t1
+  ret i1 %tobool
+}