Rollup merge of #41378 - eddyb:byval-is-not-like-sret, r=arielb1

rustc_trans: do not treat byval as using up registers.

Perhaps not that well-documented, `byval` pointer arguments *are not* the same as pointer arguments used by pass-by-ref, but rather the pointer is only used by LLVM to pass the *contents* on the stack.

Fixes #41375.

Author: frewsxcv

src/librustc_trans/cabi_x86_64.rs

@@ -229,12 +229,12 @@ pub fn compute_abi_info<'a, 'tcx>(ccx: &CrateContext<'a, 'tcx>, fty: &mut FnType
         };
 
         if in_mem {
-            // `sret` / `byval` parameter thus one less integer register available
-            int_regs -= 1;
-
             arg.make_indirect(ccx);
             if is_arg {
                 arg.attrs.set(ArgAttribute::ByVal);
+            } else {
+                // `sret` parameter thus one less integer register available
+                int_regs -= 1;
             }
         } else {
             // split into sized chunks passed individually

src/test/run-make/extern-fn-struct-passing-abi/test.c

@@ -137,6 +137,21 @@ void byval_rect_with_float(int32_t a, int32_t b, float c, int32_t d,
     assert(s.d == 556);
 }
 
+// System V x86_64 ABI:
+// a, b, d, e, f should be byval pointer (on the stack)
+// g passed via register (fixes #41375)
+//
+// Win64 ABI:
+// a, b, d, e, f, g should be byval pointer
+void byval_rect_with_many_huge(struct Huge a, struct Huge b, struct Huge c,
+                               struct Huge d, struct Huge e, struct Huge f,
+                               struct Rect g) {
+    assert(g.a == 123);
+    assert(g.b == 456);
+    assert(g.c == 789);
+    assert(g.d == 420);
+}
+
 // System V x86_64 & Win64 ABI:
 // a, b should be in registers
 // s should be split across 2 integer registers

src/test/run-make/extern-fn-struct-passing-abi/test.rs

@@ -64,6 +64,8 @@ extern {
 
     fn byval_rect_with_float(a: i32, b: i32, c: f32, d: i32, e: i32, f: i32, s: Rect);
 
+    fn byval_rect_with_many_huge(a: Huge, b: Huge, c: Huge, d: Huge, e: Huge, f: Huge, g: Rect);
+
     fn split_rect(a: i32, b: i32, s: Rect);
 
     fn split_rect_floats(a: f32, b: f32, s: FloatRect);
@@ -95,6 +97,12 @@ fn main() {
         byval_many_rect(1, 2, 3, 4, 5, 6, s);
         byval_rect_floats(1., 2., 3., 4., 5., 6., 7., s, u);
         byval_rect_with_float(1, 2, 3.0, 4, 5, 6, s);
+        byval_rect_with_many_huge(v, v, v, v, v v, Rect {
+            a: 123,
+            b: 456,
+            c: 789,
+            d: 420
+        });
         split_rect(1, 2, s);
         split_rect_floats(1., 2., u);
         split_rect_with_floats(1, 2, 3.0, 4, 5.0, 6, s);