emit-rtl: Allow extra checks for paradoxical subregs [PR119966]

When a paradoxical subreg is detected, validate_subreg exits early, thus
skipping the important checks later in the function.

Fix by continuing with the checks instead of declaring early that the
paradoxical subreg is valid.

One of the newly allowed subsequent checks needed to be disabled for
paradoxical subregs.  It turned out that combine attempts to create
a paradoxical subreg of mem even for strict-alignment targets.
That is invalid and should eventually be rejected, but is
temporarily left allowed to prevent regressions for
armv8l-unknown-linux-gnueabihf.  See PR120329 for more details.

Tests I did:
 - No regressions were found for C and C++ for the following targets:
   - native x86_64-pc-linux-gnu
   - cross riscv64-unknown-linux-gnu
   - cross riscv32-none-elf
 - Sanity checked armv8l-unknown-linux-gnueabihf by cross-building
   up to including libgcc.  Linaro CI bot further confirmed there
   are no regressions.
 - Sanity checked powerpc64-unknown-linux-gnu by building native
   toolchain, but I could not setup qemu-user for DejaGnu testing.

	PR target/119966

gcc/ChangeLog:

	* emit-rtl.cc (validate_subreg): Do not exit immediately for
	paradoxical subregs.  Filter subsequent tests which are
	not valid for paradoxical subregs.

Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>

diff --git a/gcc/emit-rtl.cc b/gcc/emit-rtl.cc
index fc7b6c7e2977f4455e8018446dfd6196dded6aee..6c838d7afcc65e98be84dee3ba1df6bb5a71455b 100644 (file)
--- a/gcc/emit-rtl.cc
+++ b/gcc/emit-rtl.cc
@@ -969,10 +969,10 @@ validate_subreg (machine_mode omode, machine_mode imode,
     }
 
   /* Paradoxical subregs must have offset zero.  */
-  if (maybe_gt (osize, isize))
-    return known_eq (offset, 0U);
+  if (maybe_gt (osize, isize) && !known_eq (offset, 0U))
+    return false;
 
-  /* This is a normal subreg.  Verify that the offset is representable.  */
+  /* Verify that the offset is representable.  */
 
   /* For hard registers, we already have most of these rules collected in
      subreg_offset_representable_p.  */
@@ -988,9 +988,13 @@ validate_subreg (machine_mode omode, machine_mode imode,
 
       return subreg_offset_representable_p (regno, imode, offset, omode);
     }
-  /* Do not allow SUBREG with stricter alignment than the inner MEM.  */
+  /* Do not allow normal SUBREG with stricter alignment than the inner MEM.
+
+     PR120329: Combine can create paradoxical mem subregs even for
+     strict-alignment targets.  Allow it until combine is fixed.  */
   else if (reg && MEM_P (reg) && STRICT_ALIGNMENT
-          && MEM_ALIGN (reg) < GET_MODE_ALIGNMENT (omode))
+          && MEM_ALIGN (reg) < GET_MODE_ALIGNMENT (omode)
+          && known_le (osize, isize))
     return false;
 
   /* The outer size must be ordered wrt the register size, otherwise
@@ -999,7 +1003,7 @@ validate_subreg (machine_mode omode, machine_mode imode,
   if (!ordered_p (osize, regsize))
     return false;
 
-  /* For pseudo registers, we want most of the same checks.  Namely:
+  /* For normal pseudo registers, we want most of the same checks.  Namely:
 
      Assume that the pseudo register will be allocated to hard registers
      that can hold REGSIZE bytes each.  If OSIZE is not a multiple of REGSIZE,
@@ -1008,8 +1012,15 @@ validate_subreg (machine_mode omode, machine_mode imode,
      otherwise it is at the lowest offset.
 
      Given that we've already checked the mode and offset alignment,
-     we only have to check subblock subregs here.  */
+     we only have to check subblock subregs here.
+
+     For paradoxical little-endian registers, this check is redundant.  The
+     offset has already been validated to be zero.
+
+     For paradoxical big-endian registers, this check is not valid
+     because the offset is zero.  */
   if (maybe_lt (osize, regsize)
+      && known_le (osize, isize)
       && ! (lra_in_progress && (FLOAT_MODE_P (imode) || FLOAT_MODE_P (omode))))
     {
       /* It is invalid for the target to pick a register size for a mode