[PATCH 4/5] sim/erc32: avoid dereferencing type-punned pointer warnings

Andrew Burgess via Gdb-patches
2022-10-12T12:39:38Z

When building the erc32 simulator I get a few warnings like this:

/tmp/build/sim/../../src/sim/erc32/exec.c:1377:21: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
1377 | sregs->fs[rd] = *((float32 *) & ddata[0]);
| ~^~~~~~~~~~~~~~~~~~~~~~~

The type of '& ddata[0]' will be 'uint32_t *', which is what triggers
the warning.

This commit uses an intermediate pointer of type 'char *' when
performing the type-punning, which is well-defined behaviour, and will
silence the above warning.

With this change, I now see no warnings when compiling exec.c, which
means that the line in Makefile.in that disables -Werror can be
removed.

There should be no change in behaviour after this commit.
---
sim/erc32/Makefile.in | 3 ---
sim/erc32/exec.c | 12 +++++++++---
2 files changed, 9 insertions(+), 6 deletions(-)

diff --git a/sim/erc32/Makefile.in b/sim/erc32/Makefile.in
index 786ae1dcc7b..41830aab726 100644
--- a/sim/erc32/Makefile.in
+++ b/sim/erc32/Makefile.in
@@ -32,9 +32,6 @@ SIM_EXTRA_CLEAN = clean-sis
# behaviour of UART interrupt routines ...
SIM_EXTRA_CFLAGS += -DFAST_UART -I$(srcroot)

-# Some modules don't build cleanly yet.
-exec.o: SIM_WERROR_CFLAGS =
-
## COMMON_POST_CONFIG_FRAG

# `sis' doesn't need interf.o.
diff --git a/sim/erc32/exec.c b/sim/erc32/exec.c
index ef93692e7a2..af9ad9ea9ab 100644
--- a/sim/erc32/exec.c
+++ b/sim/erc32/exec.c
@@ -1345,7 +1345,8 @@ dispatch_instruction(struct pstate *sregs)
if (mexc) {
sregs->trap = TRAP_DEXC;
} else {
- sregs->fs[rd] = *((float32 *) & data);
+ char *ptr = (char *) &data;
+ sregs->fs[rd] = *((float32 *) ptr);
}
break;
case LDDF:
@@ -1371,13 +1372,18 @@ dispatch_instruction(struct pstate *sregs)
if (mexc) {
sregs->trap = TRAP_DEXC;
} else {
+ char *ptr;
+
rd &= 0x1E;
sregs->flrd = rd;
- sregs->fs[rd] = *((float32 *) & ddata[0]);
+
+ ptr = (char *) &ddata[0];
+ sregs->fs[rd] = *((float32 *) ptr);
#ifdef STAT
sregs->nload++; /* Double load counts twice */
#endif
- sregs->fs[rd + 1] = *((float32 *) & ddata[1]);
+ ptr = (char *) &ddata[1];
+ sregs->fs[rd + 1] = *((float32 *) ptr);
sregs->ltime = ebase.simtime + sregs->icnt + FLSTHOLD +
sregs->hold + sregs->fhold;
}
--
2.25.4