InstCombineCast AShr transformation

[marksl]

Summary

Transform:

%shr26 = lshr i32 %0, 16                        ; [#uses=1 type=i32]
%conv4 = trunc i32 %shr26 to i16                ; [#uses=1 type=i16]
%3 = sext i16 %conv4 to i32                     ; [#uses=N type=i32]

To:

%3 = ashr i32 %0, 16

[majnemer]

This needs a testcase.

InstCombineCasts.cpp

1258–1262 ↗

(On Diff #95802)

Please use more regular register names and elide the commends.

1263 ↗

(On Diff #95802)

I'd recommend using m_APInt so that it will match against vectors.

1267 ↗

(On Diff #95802)

This will explode if the ConstantInt is bigger than an i64

[marksl]

Added test case

[marksl]

Now with transform and test case.

[efriedma]

cast.ll

1443

This testcase passes on trunk. What are you trying to fix?

[marksl]

This does not appear to be an issue on the trunk.

cast.ll

 }
 
 define <2 x i32> @test90() {
 ; CHECK-LABEL: @test90(
 ; CHECK: ret <2 x i32> <i32 0, i32 15360>
   %tmp6 = bitcast <4 x half> <half undef, half undef, half undef, half 0xH3C00> to <2 x i32>
   ret <2 x i32> %tmp6
 }
+
+define i32 @test91(i32 %i) {
+; CHECK-LABEL: @test91(
+; CHECK: ashr
+; CHECK: ret
+  %a = lshr i32 %i, 16
+  %b = trunc i32 %a to i16
+  %c = sext i16 %b to i32
+  ret i32 %c
+}