From fb7060d3c2e8ce4d7de5560adf8ec4a26fc5f6e8 Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Sun, 30 Jan 2022 16:23:31 -0700
Subject: stage2: implement shl_exact and shr_exact

These produce an undefined value when one bits are shifted out.

New AIR instruction: shr_exact.
---
 src/codegen/c.zig             |  2 +-
 src/codegen/llvm.zig          | 20 +++++++++++++++-----
 src/codegen/llvm/bindings.zig |  6 ++++++
 3 files changed, 22 insertions(+), 6 deletions(-)

(limited to 'src/codegen')

diff --git a/src/codegen/c.zig b/src/codegen/c.zig
index 5dc1f26a6b..299581cd48 100644
--- a/src/codegen/c.zig
+++ b/src/codegen/c.zig
@@ -1470,7 +1470,7 @@ fn genBody(f: *Function, body: []const Air.Inst.Index) error{ AnalysisFail, OutO
             .bit_and         => try airBinOp(f, inst, " & "),
             .bit_or          => try airBinOp(f, inst, " | "),
             .xor             => try airBinOp(f, inst, " ^ "),
-            .shr             => try airBinOp(f, inst, " >> "),
+            .shr, .shr_exact => try airBinOp(f, inst, " >> "),
             .shl, .shl_exact => try airBinOp(f, inst, " << "),
             .not             => try airNot  (f, inst),
 
diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index df64c0c912..81742d4866 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -2047,7 +2047,8 @@ pub const FuncGen = struct {
                 .bit_and, .bool_and => try self.airAnd(inst),
                 .bit_or, .bool_or   => try self.airOr(inst),
                 .xor                => try self.airXor(inst),
-                .shr                => try self.airShr(inst),
+                .shr                => try self.airShr(inst, false),
+                .shr_exact          => try self.airShr(inst, true),
 
                 .cmp_eq  => try self.airCmp(inst, .eq),
                 .cmp_gt  => try self.airCmp(inst, .gt),
@@ -3633,7 +3634,7 @@ pub const FuncGen = struct {
         return self.builder.buildUShlSat(lhs, casted_rhs, "");
     }
 
-    fn airShr(self: *FuncGen, inst: Air.Inst.Index) !?*const llvm.Value {
+    fn airShr(self: *FuncGen, inst: Air.Inst.Index, is_exact: bool) !?*const llvm.Value {
         if (self.liveness.isUnused(inst))
             return null;
         const bin_op = self.air.instructions.items(.data)[inst].bin_op;
@@ -3645,11 +3646,20 @@ pub const FuncGen = struct {
             self.builder.buildZExt(rhs, try self.dg.llvmType(lhs_type), "")
         else
             rhs;
+        const is_signed_int = self.air.typeOfIndex(inst).isSignedInt();
 
-        if (self.air.typeOfIndex(inst).isSignedInt()) {
-            return self.builder.buildAShr(lhs, casted_rhs, "");
+        if (is_exact) {
+            if (is_signed_int) {
+                return self.builder.buildAShrExact(lhs, casted_rhs, "");
+            } else {
+                return self.builder.buildLShrExact(lhs, casted_rhs, "");
+            }
         } else {
-            return self.builder.buildLShr(lhs, casted_rhs, "");
+            if (is_signed_int) {
+                return self.builder.buildAShr(lhs, casted_rhs, "");
+            } else {
+                return self.builder.buildLShr(lhs, casted_rhs, "");
+            }
         }
     }
 
diff --git a/src/codegen/llvm/bindings.zig b/src/codegen/llvm/bindings.zig
index 42c88d1351..fa285bd291 100644
--- a/src/codegen/llvm/bindings.zig
+++ b/src/codegen/llvm/bindings.zig
@@ -548,6 +548,12 @@ pub const Builder = opaque {
     pub const buildAShr = LLVMBuildAShr;
     extern fn LLVMBuildAShr(*const Builder, LHS: *const Value, RHS: *const Value, Name: [*:0]const u8) *const Value;
 
+    pub const buildLShrExact = ZigLLVMBuildLShrExact;
+    extern fn ZigLLVMBuildLShrExact(*const Builder, LHS: *const Value, RHS: *const Value, Name: [*:0]const u8) *const Value;
+
+    pub const buildAShrExact = ZigLLVMBuildAShrExact;
+    extern fn ZigLLVMBuildAShrExact(*const Builder, LHS: *const Value, RHS: *const Value, Name: [*:0]const u8) *const Value;
+
     pub const buildShl = LLVMBuildShl;
     extern fn LLVMBuildShl(*const Builder, LHS: *const Value, RHS: *const Value, Name: [*:0]const u8) *const Value;
 
-- 
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