soft-fp: Fix _FP_FMA when product is zero and third argument is finite (bug 17932)
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Message ID alpine.DEB.2.10.1502060228160.3494@digraph.polyomino.org.uk
State New
Headers show

Commit Message

Joseph Myers Feb. 6, 2015, 2:30 a.m. UTC
soft-fp's _FP_FMA fails to set the result's exponent for cases where
the result of the multiplication is 0, yielding incorrect (arbitrary,
depending on uninitialized values) results for those cases.  This
affects libm for architectures using soft-fp to implement fma.  This
patch adds the exponent setting and tests for this case.

Tested for ARM soft-float (which uses soft-fp fma), x86_64 and x86 (to
verify not introducing new libm test failures there).  Will commit
when master reopens.

(This bug showed up in testing my patch to move the Linux kernel to
current soft-fp.  math/Makefile has "override CFLAGS +=
-Wno-uninitialized" which would have stopped compiler warnings from
showing up this problem, although I wouldn't be surprised if removing
that shows spurious warnings from this code, if the compiler fails to
follow that various cases where the exponent is uninitialized don't
need it initialized because the class is set to a value meaning the
uninitialized exponent isn't used.)

(auto-libm-test-out diffs omitted below.)

2015-02-06  Joseph Myers  <joseph@codesourcery.com>

	[BZ #17932]
	* soft-fp/op-common.h (_FP_FMA): Set exponent of result in case
	where multiplication results in zero and third argument is finite
	and nonzero.
	* math/auto-libm-test-in: Add more tests of fma.
	* math/auto-libm-test-out: Regenerated.

Patch
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diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index 8db3eb6..ad43113 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -1024,6 +1024,16 @@  fma 1.0 -1.0 1.0
 fma -1.0 1.0 1.0
 fma -1.0 -1.0 -1.0
 
+fma 0 0 1
+fma 0 0 2
+fma 0 0 max
+fma 0 1 1
+fma 1 0 1
+fma 0 1 2
+fma 1 0 2
+fma 0 1 max
+fma 1 0 max
+
 # Bug 6801: errno setting may be missing.
 fma min min 0 missing-errno
 fma min min -0 missing-errno
diff --git a/soft-fp/op-common.h b/soft-fp/op-common.h
index 6b8b52f..d115f58 100644
--- a/soft-fp/op-common.h
+++ b/soft-fp/op-common.h
@@ -1116,6 +1116,7 @@ 
 	  R##_s = Z##_s;						\
 	  _FP_FRAC_COPY_##wc (R, Z);					\
 	  R##_c = Z##_c;						\
+	  R##_e = Z##_e;						\
 	  break;							\
 									\
 	case _FP_CLS_COMBINE (FP_CLS_INF, FP_CLS_INF):			\