diff --git a/kernel/fmt.cc b/kernel/fmt.cc
index cbf2d12e9..d1c6b8ac9 100644
--- a/kernel/fmt.cc
+++ b/kernel/fmt.cc
@@ -832,7 +832,7 @@ std::string Fmt::render() const
 						}
 					} else log_abort();
 					if (part.justify == FmtPart::NUMERIC && part.group && part.padding == '0') {
-						int group_size = part.base == 10 ? 3 : 4;
+						size_t group_size = part.base == 10 ? 3 : 4;
 						while (prefix.size() + buf.size() < part.width) {
 							if (buf.size() % (group_size + 1) == group_size)
 								buf += '_';
diff --git a/kernel/satgen.cc b/kernel/satgen.cc
index baaf22d1f..a6db78868 100644
--- a/kernel/satgen.cc
+++ b/kernel/satgen.cc
@@ -1228,7 +1228,7 @@ bool SatGen::importCell(RTLIL::Cell *cell, int timestep)
 			if (ff.has_srst && ff.has_ce && ff.ce_over_srst) {
 				int srst = importDefSigSpec(ff.sig_srst, timestep-1).at(0);
 				std::vector<int> rval = importDefSigSpec(ff.val_srst, timestep-1);
-				int undef_srst;
+				int undef_srst = -1;
 				std::vector<int> undef_rval;
 				if (model_undef) {
 					undef_srst = importUndefSigSpec(ff.sig_srst, timestep-1).at(0);
@@ -1242,7 +1242,7 @@ bool SatGen::importCell(RTLIL::Cell *cell, int timestep)
 			if (ff.has_ce) {
 				int ce = importDefSigSpec(ff.sig_ce, timestep-1).at(0);
 				std::vector<int> old_q = importDefSigSpec(ff.sig_q, timestep-1);
-				int undef_ce;
+				int undef_ce = -1;
 				std::vector<int> undef_old_q;
 				if (model_undef) {
 					undef_ce = importUndefSigSpec(ff.sig_ce, timestep-1).at(0);
@@ -1256,7 +1256,7 @@ bool SatGen::importCell(RTLIL::Cell *cell, int timestep)
 			if (ff.has_srst && !(ff.has_ce && ff.ce_over_srst)) {
 				int srst = importDefSigSpec(ff.sig_srst, timestep-1).at(0);
 				std::vector<int> rval = importDefSigSpec(ff.val_srst, timestep-1);
-				int undef_srst;
+				int undef_srst = -1;
 				std::vector<int> undef_rval;
 				if (model_undef) {
 					undef_srst = importUndefSigSpec(ff.sig_srst, timestep-1).at(0);
diff --git a/libs/fst/fstapi.cc b/libs/fst/fstapi.cc
index 592b5cae6..14c981a42 100644
--- a/libs/fst/fstapi.cc
+++ b/libs/fst/fstapi.cc
@@ -5536,7 +5536,9 @@ char *fstReaderGetValueFromHandleAtTime(void *ctx, uint64_t tim, fstHandle facid
     fst_off_t blkpos = 0, prev_blkpos;
     uint64_t beg_tim, end_tim, beg_tim2, end_tim2;
     int sectype;
+#ifdef FST_DEBUG
     unsigned int secnum = 0;
+#endif
     uint64_t seclen;
     uint64_t tsec_uclen = 0, tsec_clen = 0;
     uint64_t tsec_nitems;
@@ -5620,7 +5622,9 @@ char *fstReaderGetValueFromHandleAtTime(void *ctx, uint64_t tim, fstHandle facid
         }
 
         blkpos += seclen;
+#ifdef FST_DEBUG
         secnum++;
+#endif
     }
 
     xc->rvat_beg_tim = beg_tim;
diff --git a/passes/techmap/abc.cc b/passes/techmap/abc.cc
index b33e1a17d..07f9ee45d 100644
--- a/passes/techmap/abc.cc
+++ b/passes/techmap/abc.cc
@@ -1718,7 +1718,7 @@ struct AbcPass : public Pass {
 			show_tempdir = true;
 		}
 
-		size_t argidx, g_argidx;
+		size_t argidx, g_argidx = -1;
 		bool g_arg_from_cmd = false;
 #if defined(__wasm)
 		const char *pwd = ".";