[llvm] [SimplifyCFG] Make i1 legal to build lookup tables (PR #87431)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Arthur Carcano (krtab)

<details>
<summary>Changes</summary>

This is my attempt to fix the LLVM issue leading to https://github.com/rust-lang/rust/issues/123305 and my first contribution to LLVM.

`i1` types are not considered to simplify switches because they are `!isTypeLegalForLookupTable`. This hardcode an exception but other ways may make more sense.

I guess I should add a test? Could you point to me where?

---
Full diff: https://github.com/llvm/llvm-project/pull/87431.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (+2) 
- (added) llvm/test/Transforms/SimplifyCFG/fold-i1.ll (+64) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 55bbffb18879fb..af6ace5675bf52 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -6448,6 +6448,8 @@ static bool isTypeLegalForLookupTable(Type *Ty, const TargetTransformInfo &TTI,
   // on ABI alignment and padding in the table to allow the load to be widened.
   // Or we could widen the constants and truncate the load.
   unsigned BitWidth = IT->getBitWidth();
+  if (BitWidth == 1)
+    return true;
   return BitWidth >= 8 && isPowerOf2_32(BitWidth) &&
          DL.fitsInLegalInteger(IT->getBitWidth());
 }
diff --git a/llvm/test/Transforms/SimplifyCFG/fold-i1.ll b/llvm/test/Transforms/SimplifyCFG/fold-i1.ll
new file mode 100644
index 00000000000000..444f51fb2438bc
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/fold-i1.ll
@@ -0,0 +1,64 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt < %s -S -passes='simplifycfg<switch-to-lookup>' | FileCheck %s
+
+target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define { i1, i8 } @f(i8 noundef %character) unnamed_addr {
+; CHECK-LABEL: define { i1, i8 } @f(
+; CHECK-SAME: i8 noundef [[CHARACTER:%.*]]) unnamed_addr {
+; CHECK-NEXT:  start:
+; CHECK-NEXT:    [[SWITCH_TABLEIDX:%.*]] = sub i8 [[CHARACTER]], 48
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp ult i8 [[SWITCH_TABLEIDX]], 9
+; CHECK-NEXT:    [[_0_SROA_11_0:%.*]] = select i1 [[TMP2]], i8 [[SWITCH_TABLEIDX]], i8 undef
+; CHECK-NEXT:    [[_0_SROA_0_0:%.*]] = select i1 [[TMP2]], i1 true, i1 false
+; CHECK-NEXT:    [[TMP0:%.*]] = insertvalue { i1, i8 } poison, i1 [[_0_SROA_0_0]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = insertvalue { i1, i8 } [[TMP0]], i8 [[_0_SROA_11_0]], 1
+; CHECK-NEXT:    ret { i1, i8 } [[TMP1]]
+;
+start:
+  switch i8 %character, label %bb11 [
+  i8 48, label %bb2
+  i8 49, label %bb3
+  i8 50, label %bb4
+  i8 51, label %bb5
+  i8 52, label %bb6
+  i8 53, label %bb7
+  i8 54, label %bb8
+  i8 55, label %bb9
+  i8 56, label %bb10
+  ]
+
+bb2:
+  br label %bb11
+
+bb3:
+  br label %bb11
+
+bb4:
+  br label %bb11
+
+bb5:
+  br label %bb11
+
+bb6:
+  br label %bb11
+
+bb7:
+  br label %bb11
+
+bb8:
+  br label %bb11
+
+bb9:
+  br label %bb11
+
+bb10:
+  br label %bb11
+
+bb11:
+  %_0.sroa.11.0 = phi i8 [ 8, %bb10 ], [ 7, %bb9 ], [ 6, %bb8 ], [ 5, %bb7 ], [ 4, %bb6 ], [ 3, %bb5 ], [ 2, %bb4 ], [ 1, %bb3 ], [ 0, %bb2 ], [ undef, %start ]
+  %_0.sroa.0.0 = phi i1 [ true, %bb10 ], [ true, %bb9 ], [ true, %bb8 ], [ true, %bb7 ], [ true, %bb6 ], [ true, %bb5 ], [ true, %bb4 ], [ true, %bb3 ], [ true, %bb2 ], [ false, %start ]
+  %0 = insertvalue { i1, i8 } poison, i1 %_0.sroa.0.0, 0
+  %1 = insertvalue { i1, i8 } %0, i8 %_0.sroa.11.0, 1
+  ret { i1, i8 } %1
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/87431