[llvm] [InstCombine] Propagate exact flags in shift-combine transforms (PR #88340)

[Nikita Popov via llvm-commits]

================
@@ -433,6 +433,34 @@ entry:
   ret i32 %i10
 }
 
+define i32 @test29_exact(i64 %d18) {
+; CHECK-LABEL: @test29_exact(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SUM_SHIFT:%.*]] = lshr exact i64 [[D18:%.*]], 63
+; CHECK-NEXT:    [[I101:%.*]] = trunc nuw nsw i64 [[SUM_SHIFT]] to i32
+; CHECK-NEXT:    ret i32 [[I101]]
+;
+entry:
+  %i916 = lshr exact i64 %d18, 32
+  %i917 = trunc i64 %i916 to i32
+  %i10 = lshr exact i32 %i917, 31
+  ret i32 %i10
+}
+
+define i32 @test29_exact_fail(i64 %d18) {
+; CHECK-LABEL: @test29_exact_fail(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SUM_SHIFT:%.*]] = lshr i64 [[D18:%.*]], 63
+; CHECK-NEXT:    [[I101:%.*]] = trunc nuw nsw i64 [[SUM_SHIFT]] to i32
+; CHECK-NEXT:    ret i32 [[I101]]
+;
+entry:
+  %i916 = lshr exact i64 %d18, 32
+  %i917 = trunc i64 %i916 to i32
+  %i10 = lshr i32 %i917, 31
+  ret i32 %i10
+}
----------------
nikic wrote:

Also add a variant where the exact is only on the last shift, rather than only on the first.

https://github.com/llvm/llvm-project/pull/88340