[compiler-rt][RISC-V] ILP32E/LP64E Save/Restore Grouping (llvm#95398)

This changes the save/restore procedures to save/restore registers one
by one - to match the stack alignment for the ILP32E/LP64E ABIs, rather
than the larger batches of the conventional ABIs. The implementations of
the save routines are not tail-shared, to reduce the number of
instructions. I think this also helps code size but I need to check this
again.

I would expect (but haven't measured) that the majority of functions
compiled for the ILP32E/LP64E ABIs will in fact use both callee-saved
registers, and therefore there are still savings to be had, but I think
those can come later, with more data (especially if those changes are
just to the instruction sequences we use to save the registers, rather
than the number and alignment of how this is done).

This is a potential break for all of the ILP32E/LP64E ABI - we may
instead have to teach the compiler to emit the CFI information correctly
for the grouping we already have implemented (because that grouping
matches GCC). It depends on how intentional we think the grouping is in
the original ILP32E/LP64E save/restore implementation was, and whether
we think we can fix that now.

Author: lenary

compiler-rt/lib/builtins/riscv/restore.S

@@ -14,9 +14,12 @@
 // them falling through into each other and don't want the linker to
 // accidentally split them up, garbage collect, or reorder them.
 //
-// The entry points are grouped up into 2s for rv64 and 4s for rv32 since this
-// is the minimum grouping which will maintain the required 16-byte stack
-// alignment.
+// For the conventional ABIs, entry points are grouped up into 2s for rv64 and
+// 4s for rv32 since this is the minimum grouping which will maintain the
+// required 16-byte stack alignment.
+//
+// For the ilp32e/lp64e abis, entry points are grouped into 1s, since this is
+// the minimum grouping which will maintain the required 4-byte stack alignment.
 
   .text
 
@@ -92,17 +95,23 @@ __riscv_restore_0:
 
   .globl  __riscv_restore_2
   .type   __riscv_restore_2,@function
+__riscv_restore_2:
+  lw      s1, 0(sp)
+  addi    sp, sp, 4
+  // fallthrough into __riscv_restore_1/0
+
   .globl  __riscv_restore_1
   .type   __riscv_restore_1,@function
+__riscv_restore_1:
+  lw      s0, 0(sp)
+  addi    sp, sp, 4
+  // fallthrough into __riscv_restore_0
+
   .globl  __riscv_restore_0
   .type   __riscv_restore_0,@function
-__riscv_restore_2:
-__riscv_restore_1:
 __riscv_restore_0:
-  lw      s1,  0(sp)
-  lw      s0,  4(sp)
-  lw      ra,  8(sp)
-  addi    sp, sp, 12
+  lw      ra, 0(sp)
+  addi    sp, sp, 4
   ret
 
 #endif
@@ -188,17 +197,23 @@ __riscv_restore_0:
 
   .globl  __riscv_restore_2
   .type   __riscv_restore_2,@function
+__riscv_restore_2:
+  ld      s1, 0(sp)
+  addi    sp, sp, 8
+  // fallthrough into __riscv_restore_1/0
+
   .globl  __riscv_restore_1
   .type   __riscv_restore_1,@function
+__riscv_restore_1:
+  ld      s0, 0(sp)
+  addi    sp, sp, 8
+  // fallthrough into __riscv_restore_0
+
   .globl  __riscv_restore_0
   .type   __riscv_restore_0,@function
-__riscv_restore_2:
-__riscv_restore_1:
 __riscv_restore_0:
-  ld      s1,  0(sp)
-  ld      s0,  8(sp)
-  ld      ra,  16(sp)
-  addi    sp, sp, 24
+  ld      ra, 0(sp)
+  addi    sp, sp, 8
   ret
 
 #endif

compiler-rt/lib/builtins/riscv/save.S

@@ -98,19 +98,28 @@ __riscv_save_0:
 
   .globl  __riscv_save_2
   .type   __riscv_save_2,@function
-  .globl  __riscv_save_1
-  .type   __riscv_save_1,@function
-  .globl  __riscv_save_0
-  .type   __riscv_save_0,@function
 __riscv_save_2:
-__riscv_save_1:
-__riscv_save_0:
   addi    sp, sp, -12
   sw      s1,  0(sp)
   sw      s0,  4(sp)
   sw      ra,  8(sp)
   jr      t0
 
+  .globl  __riscv_save_1
+  .type   __riscv_save_1,@function
+__riscv_save_1:
+  addi    sp, sp, -8
+  sw      s0,  0(sp)
+  sw      ra,  4(sp)
+  jr      t0
+
+  .globl  __riscv_save_0
+  .type   __riscv_save_0,@function
+__riscv_save_0:
+  addi    sp, sp, -4
+  sw      ra,  0(sp)
+  jr      t0
+
 #endif
 
 #elif __riscv_xlen == 64
@@ -208,18 +217,27 @@ __riscv_save_0:
 
   .globl  __riscv_save_2
   .type   __riscv_save_2,@function
+__riscv_save_2:
+  addi    sp, sp, -24
+  sw      s1, 0(sp)
+  sw      s0, 8(sp)
+  sw      ra, 16(sp)
+  jr      t0
+
   .globl  __riscv_save_1
   .type   __riscv_save_1,@function
+__riscv_save_1:
+  addi    sp, sp, -16
+  sw      s0, 0(sp)
+  sw      ra, 8(sp)
+  jr      t0
+
   .globl  __riscv_save_0
   .type   __riscv_save_0,@function
-__riscv_save_2:
-__riscv_save_1:
 __riscv_save_0:
-  addi   sp, sp, -24
-  sd     s1, 0(sp)
-  sd     s0, 8(sp)
-  sd     ra, 16(sp)
-  jr     t0
+  addi    sp, sp, -8
+  sw      ra,  0(sp)
+  jr      t0
 
 #endif