rustc_codegen_ssa: Better code generation for niche discriminants.

In some cases we can avoid arithmetic before checking whether a niche
represents an untagged variant.

This is relevant to #101872

1 files changed, 114 insertions, 0 deletions

diff --git a/src/test/ui/enum-discriminant/get_discr.rs b/src/test/ui/enum-discriminant/get_discr.rs
new file mode 100644
index 00000000000..71eea4e0f78
--- /dev/null
+++ b/src/test/ui/enum-discriminant/get_discr.rs
@@ -0,0 +1,114 @@
+// run-pass
+
+// Now that there are several variations on the code generated in
+// `codegen_get_discr`, let's make sure the various cases yield the correct
+// result.
+
+// To get the discriminant of an E<X1> value, there are no shortcuts - we must
+// do the full algorithm.
+#[repr(u8)]
+pub enum X1 {
+    _1 = 1, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16,
+    _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32,
+    _33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48,
+    _49, _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, _61, _62, _63, _64,
+    _65, _66, _67, _68, _69, _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _80,
+    _81, _82, _83, _84, _85, _86, _87, _88, _89, _90, _91, _92, _93, _94, _95, _96,
+    _97, _98, _99, _100, _101, _102, _103, _104, _105, _106, _107, _108, _109, _110, _111, _112,
+    _113, _114, _115, _116, _117, _118, _119, _120, _121, _122, _123, _124, _125, _126, _127, _128,
+    _129, _130, _131, _132, _133, _134, _135, _136, _137, _138, _139, _140, _141, _142, _143, _144,
+    _145, _146, _147, _148, _149, _150, _151, _152, _153, _154, _155, _156, _157, _158, _159, _160,
+    _161, _162, _163, _164, _165, _166, _167, _168, _169, _170, _171, _172, _173, _174, _175, _176,
+    _177, _178, _179, _180, _181, _182, _183, _184, _185, _186, _187, _188, _189, _190, _191, _192,
+    _193, _194, _195, _196, _197, _198, _199, _200, _201, _202, _203, _204, _205, _206, _207, _208,
+    _209, _210, _211, _212, _213, _214, _215, _216, _217, _218, _219, _220, _221, _222, _223, _224,
+    _225, _226, _227, _228, _229, _230, _231, _232, _233, _234, _235, _236, _237, _238, _239, _240,
+    _241, _242, _243, _244, _245, _246, _247, _248, _249, _250, _251, _252, _253, _254,
+}
+
+#[repr(i8)]
+pub enum X2 {
+    _1 = -1, _2 = 0, _3 = 1,
+}
+
+#[repr(i8)]
+pub enum X3 {
+    _1 = -128, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16,
+    _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32,
+    _33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48,
+    _49, _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, _61, _62, _63, _64,
+    _65, _66, _67, _68, _69, _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _80,
+    _81, _82, _83, _84, _85, _86, _87, _88, _89, _90, _91, _92, _93, _94, _95, _96,
+    _97, _98, _99, _100, _101, _102, _103, _104, _105, _106, _107, _108, _109, _110, _111, _112,
+    _113, _114, _115, _116, _117, _118, _119, _120, _121, _122, _123, _124, _125, _126, _127, _128,
+    _129, _130, _131, _132, _133, _134, _135, _136, _137, _138, _139, _140, _141, _142, _143, _144,
+    _145, _146, _147, _148, _149, _150, _151, _152, _153, _154, _155, _156, _157, _158, _159, _160,
+    _161, _162, _163, _164, _165, _166, _167, _168, _169, _170, _171, _172, _173, _174, _175, _176,
+    _177, _178, _179, _180, _181, _182, _183, _184, _185, _186, _187, _188, _189, _190, _191, _192,
+    _193, _194, _195, _196, _197, _198, _199, _200, _201, _202, _203, _204, _205, _206, _207, _208,
+    _209, _210, _211, _212, _213, _214, _215, _216, _217, _218, _219, _220, _221, _222, _223, _224,
+    _225, _226, _227, _228, _229, _230, _231, _232, _233, _234, _235, _236, _237, _238, _239, _240,
+    _241, _242, _243, _244, _245, _246, _247, _248, _249, _250, _251, _252, _253, _254,
+}
+
+#[repr(i8)]
+pub enum X4 {
+    _1 = -126, _2, _3, _4, _5, _6, _7, _8, _9, _10, _11, _12, _13, _14, _15, _16,
+    _17, _18, _19, _20, _21, _22, _23, _24, _25, _26, _27, _28, _29, _30, _31, _32,
+    _33, _34, _35, _36, _37, _38, _39, _40, _41, _42, _43, _44, _45, _46, _47, _48,
+    _49, _50, _51, _52, _53, _54, _55, _56, _57, _58, _59, _60, _61, _62, _63, _64,
+    _65, _66, _67, _68, _69, _70, _71, _72, _73, _74, _75, _76, _77, _78, _79, _80,
+    _81, _82, _83, _84, _85, _86, _87, _88, _89, _90, _91, _92, _93, _94, _95, _96,
+    _97, _98, _99, _100, _101, _102, _103, _104, _105, _106, _107, _108, _109, _110, _111, _112,
+    _113, _114, _115, _116, _117, _118, _119, _120, _121, _122, _123, _124, _125, _126, _127, _128,
+    _129, _130, _131, _132, _133, _134, _135, _136, _137, _138, _139, _140, _141, _142, _143, _144,
+    _145, _146, _147, _148, _149, _150, _151, _152, _153, _154, _155, _156, _157, _158, _159, _160,
+    _161, _162, _163, _164, _165, _166, _167, _168, _169, _170, _171, _172, _173, _174, _175, _176,
+    _177, _178, _179, _180, _181, _182, _183, _184, _185, _186, _187, _188, _189, _190, _191, _192,
+    _193, _194, _195, _196, _197, _198, _199, _200, _201, _202, _203, _204, _205, _206, _207, _208,
+    _209, _210, _211, _212, _213, _214, _215, _216, _217, _218, _219, _220, _221, _222, _223, _224,
+    _225, _226, _227, _228, _229, _230, _231, _232, _233, _234, _235, _236, _237, _238, _239, _240,
+    _241, _242, _243, _244, _245, _246, _247, _248, _249, _250, _251, _252, _253, _254,
+}
+
+pub enum E<X> {
+    A(X),
+    B,
+    C,
+}
+
+pub fn match_e<X>(e: E<X>) -> u8 {
+    use E::*;
+    match e {
+        A(_) => 0,
+        B => 1,
+        C => 2,
+    }
+}
+
+fn main() {
+    assert_eq!(match_e(E::A(X1::_1)), 0);
+    assert_eq!(match_e(E::A(X1::_2)), 0);
+    assert_eq!(match_e(E::A(X1::_254)), 0);
+    assert_eq!(match_e(E::<X1>::B), 1);
+    assert_eq!(match_e(E::<X1>::C), 2);
+    assert_eq!(match_e(E::A(X2::_1)), 0);
+    assert_eq!(match_e(E::A(X2::_2)), 0);
+    assert_eq!(match_e(E::A(X2::_3)), 0);
+    assert_eq!(match_e(E::<X2>::B), 1);
+    assert_eq!(match_e(E::<X2>::C), 2);
+    assert_eq!(match_e(E::A(X3::_1)), 0);
+    assert_eq!(match_e(E::A(X3::_2)), 0);
+    assert_eq!(match_e(E::A(X3::_254)), 0);
+    assert_eq!(match_e(E::<X3>::B), 1);
+    assert_eq!(match_e(E::<X3>::C), 2);
+    assert_eq!(match_e(E::A(X4::_1)), 0);
+    assert_eq!(match_e(E::A(X4::_2)), 0);
+    assert_eq!(match_e(E::A(X4::_254)), 0);
+    assert_eq!(match_e(E::<X4>::B), 1);
+    assert_eq!(match_e(E::<X4>::C), 2);
+    assert_eq!(match_e(E::A(false)), 0);
+    assert_eq!(match_e(E::A(true)), 0);
+    assert_eq!(match_e(E::<bool>::B), 1);
+    assert_eq!(match_e(E::<bool>::C), 2);
+}